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Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California: Breeding Activities and Habitat Use—2022 Annual Report

Alexandra Houston, Lisa D. Allen, Shannon M. Mendia, Barbara E. Kus, K.P. Burnham, D.R. Anderson · U.S. Geological Survey

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Ecosystems Mission Area—Species Management Research Program

Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California: Breeding Activities and Habitat Use—2022 Annual Report

Open-File Report 2023–1040

U.S. Department of the Interior U.S. Geological Survey

Cover: Least Bell’s Vireo (Vireo bellii pusillus) nest with two vireo nestlings at the lower San Luis Rey River. Photograph by Shannon Mendia, U.S. Geological Survey, 2022.

Ecosystems Mission Area—Species Management Research Program

Open-File Report 2023–1040

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2023

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Houston, A., Allen, L.D., Mendia, S.M., and Kus, B.E., 2023, Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2022 annual report: U.S. Geological Survey Open-File Report 2023–1040, 74 p., https://doi.org/10.3133/ofr20231040. ISSN 2331-1258 (online)

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Contents Executive Summary��1 Introduction��2 Least Bell’s Vireo��2 Southwestern Willow Flycatcher��3 San Luis Rey Flood Risk Management Project Area��3 Purpose and Scope��6 Methods��7 Surveys��7 Nest Monitoring��7 Analysis of Nesting Data��8 Effects of Treatment��9 Nest Survival Analysis��10 Banding��10 Survivorship Estimates��11 Site Fidelity and Movement��12 Vegetation Study Design��12 Vireo Habitat Use Study Design��12 Vegetation Sampling��13 Vegetation Data Analysis��14 Results��15 Least Bell’s Vireo��15 Population Size and Distribution��15 Effects of Treatment on Habitat Use��18 Nest Monitoring��18 Nesting Attempts��18 Apparent Nest Success��20 Brown-headed Cowbird Parasitism��20 Reproductive Success and Productivity��22 Effects of Treatment on Vireo Reproduction��22 Nest Survival and Habitat Use��25 Nest Characteristics��26 Banded Birds��27 Emigrants��27 New Captures��27 Survivorship, Fidelity, and Movement��30 Survivorship Models—Adults-Only��30 Survivorship Models—Adults and First-Year Vireos��30 First-year Dispersal 2022��30 First-year Dispersal 2007–22��30 Adult Site Fidelity��34 Effect of Treatment on Adult Site Fidelity��34

Southwestern Willow Flycatcher��34 Population Size and Distribution��34 Habitat Characteristics��34 Vegetation Study��34 Vegetation Structure and Composition��34 Vegetation Changes 2006–22��36 Vireo Habitat Use��40 Discussion��41 Southwestern Willow Flycatcher��42 Conclusion��43 References Cited��43 Appendix 1. ��46 Appendix 2. ��50 Appendix 3. ��54 Appendix 4. ��59 Appendix 5. ��62 Appendix 6. ��63 Appendix 7. ��64 Appendix 8. ��66 Appendix 9. ��68 Appendix 10. ��72

Figures 1.

3. 4. 5. 6. 7. 8.

Image showing Least Bell’s Vireo and Southwestern Willow Flycatcher survey and monitoring sites at the San Luis Rey Flood Risk Management Project Area, California, in 2022��4 Image showing Least Bell’s Vireo territory boundary with two treatments and a section of overlap between the two treatments at the San Luis Rey Flood Risk Management Project Area, California, 2022��9 Image showing vegetation transects in the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022��13 Image showing nest-centered and territory vegetation plots sampled at the San Luis Rey Flood Risk Management Project Area, California, 2022��13 Graph showing number of Least Bell’s Vireo territories from 1984 to 2022 at the San Luis Rey Flood Risk Management Project Area, California��17 Graph showing number of first Least Bell’s Vireo nests initiated by week at the San Luis Rey Flood Risk Management Project Area, California, 2022��19 Graph showing annual median initiation dates of first nests at the San Luis Rey Flood Risk Management Project Area, California, 2006–22��19 Graph showing number of completed Least Bell’s Vireo nests that were parasitized by Brown-headed Cowbirds at the San Luis Rey Flood Risk Management Project Area, California, 2006–22��21

10. 11. 12.

13. 14.

15. 16. 17.

18.

19.

Graph showing number of completed Least Bell’s Vireo nests initiated and those that were parasitized by Brown-headed Cowbirds at the San Luis Rey Flood Risk Management Project Area, California, 2022��21 Graph showing average number of fledglings per pair at the San Luis Rey Flood Risk Management Project Area, California, 2006–22��23 Graph showing Treatment Index of monitored vireo territories within the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022��23 Graphs showing effect of Treatment Index on Least Bell’s Vireo reproductive parameters at the San Luis Rey Flood Risk Management Project Area, California, 2022��24 Graph showing average percent foliage cover by height class at the San Luis Rey Flood Risk Management Project Area, California, 2022��35 Graph showing percent of total foliage cover by vegetation type at Channel, Upper Pond, and Whelan Restoration sites at the San Luis Rey Flood Risk Management Project Area, California, 2022��36 Graph showing average percent foliage cover by height class for the Channel at the San Luis Rey Flood Risk Management Project Area, 2006–22��37 Graph showing average percent foliage cover by height class for Upper Pond at the San Luis Rey Flood Risk Management Project Area, 2006–22��38 Graph showing average percent foliage cover by height class for Whelan Restoration and Whelan Mitigation at the San Luis Rey Flood Risk Management Project Area, 2006–22��39 Graphs showing average percent foliage cover for territory plots compared to vegetation transects and nest plots compared to territory plots by height class in the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022��40 Graphs showing average percent foliage cover for territory plots compared to vegetation transects and nest plots compared to territory plots by height class in the Upper Pond at the San Luis Rey Flood Risk Management Project Area, California, 2022��40

Tables 1. 2. 3. 4. 5. 6.

Least Bell’s Vireo and Southwestern Willow Flycatcher survey sites at the San Luis Rey Flood Risk Management Project Area, California, in 2022��4 Site attributes of the Channel, Off-channel, and Restoration monitoring sites at the San Luis Rey Flood Risk Management Project Area, California, in 2022��5 Number and breeding status of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022��15 Number and distribution of Least Bell’s Vireo territories at the San Luis Rey Flood Risk Management Project Area, California, 2006–22��16 Number of Least Bell’s Vireo territories and nests monitored at the San Luis Rey Flood Risk Management Project Area, California, 2022��18 Fate of completed Least Bell’s Vireo nests at the San Luis Rey Flood Risk Management Project Area, California, 2022��20

8. 9. 10.

11.

12.

13.

14.

15. 16.

17. 18.

19.

20.

21. 22.

Number and fate of Least Bell’s Vireo nests parasitized by Brown-headed Cowbirds at the San Luis Rey Flood Risk Management Project Area, California, 2022��20 Reproductive parameters for nesting Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022��22 Productivity of nesting Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022��22 Generalized linear model results, parameter estimate, standard error, Z statistic, and P-values for Least Bell’s Vireo reproductive parameters as a function of Treatment Index within the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022��24 Logistic regression models for the effects of Treatment Index on daily nest survival of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022��25 Logistic regression models for the effects of habitat structure on daily nest survival of Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22��25 Parameter estimate, standard error, odds ratios, and 95-percent confidence intervals for models explaining the effect of habitat structure on the daily survival rate of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2006–22��26 Least Bell’s Vireo nest characteristics and results of Kruskal-Wallis and Mann-Whitney U-tests comparing Channel, Off-channel, and Restoration nests and successful versus failed nests at the San Luis Rey Flood Risk Management Project Area, California, 2022��26 Host plant species used by Least Bell’s Vireos for nesting at the San Luis Rey Flood Risk Management Project Area, California, 2022��28 Banding status of Least Bell’s Vireos detected at the San Luis Rey Flood Risk Management Project Area, California, in 2022, and those that emigrated from the Project Area, 2022��29 Number of banded adult Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022, by original year banded, age, and sex��29 Number of emigrant Least Bell’s Vireos detected outside of the Project Area that originated in the San Luis Rey Flood Risk Management Project Area, California, by original year banded, age, 2022 location, and sex��29 Survivorship models for the effects of sex, year, and precipitation on adult survival for Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22��30 Parameter estimate, standard error, odds ratios, and 95-percent confidence intervals for the top model explaining annual survivorship of adult Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22��31 Annual survivorship for adult female and adult male Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22��31 Survivorship models for the effect of age, year, and precipitation on survival of Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22��31

vii

23.

24. 25.

26.

27. 28.

Parameter estimate, standard error, odds ratios, and 95-percent confidence intervals for the top model explaining annual survivorship of Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22��32 Annual survivorship for adult and first-year Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22��32 Number of first-year Least Bell's Vireos that originated at the San Luis Rey Flood Risk Management Project Area and returned to the Project Area or returned to areas outside of the Project Area, 2007–22, by year and sex��33 Generalized linear model results, parameter estimate, standard error, t statistic, and P-values for banded Least Bell’s Vireo movement as a function of the expected Treatment Index for 2021 territories at the San Luis Rey Flood Risk Management Project Area, California, 2022��34 Habitat characteristics of transient Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area, California, 2022��35 Number of vegetation transects and sampling points at the San Luis Rey Flood Risk Management Project Area, California, 2022��35

Conversion Factors International System of Units to U.S. customary units

Multiply

To obtain

Length centimeter (cm)

0.394

inch (in)

meter (m)

3.281

foot (ft)

kilometer (km)

0.6214

mile (mi)

hectare (ha)

2.471

cubic meter per second (m3/s)

35.315

cubic feet per second (ft3/s)

kilometer per hour (km/h)

0.6214

mile per hour (mi/h)

Area acre (ac)

Flow

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C = (°F −32) / 1.8.

Datum Horizontal coordinate information is referenced to the World Geographic System of 1984 (WGS 84).

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Abbreviations AICc

Akaike’s Information Criterion for small sample sizes

DSR

daily survival rate

FNWS

Fallbrook Naval Weapons Station

GLM

generalized linear model

GPS

Global Positioning System

MCBCP

Marine Corps Base Camp Pendleton

Project Area USFWS

San Luis Rey Flood Risk Management Project Area

U.S. Fish and Wildlife Service

Executive Summary We completed four protocol surveys for Least Bell’s Vireos (Vireo bellii pusillus; vireo) during the breeding season, supplemented by weekly territory monitoring visits. We identified a total of 133 territorial male vireos; 114 were confirmed as paired, and 3 were confirmed as single males. For the remaining 16 territories, we were unable to confirm breeding status. Two transient vireos were detected in 2022. The vireo population in the Project Area increased by 9 percent from 2021 to 2022. The vireo population at Marine Corps Base Camp Pendleton also increased (4 percent), whereas the population at Marine Corps Air Station remained relatively stable (decreased from 10 pairs to 9) and the Otay River population decreased by 10 percent (2 territories). We used an index of treatment (Treatment Index) to evaluate the effect of on-going vegetation clearing on the Project Area vireo population. The Treatment Index measures the cumulative effect of vegetation treatment within a territory (since 2005) by using the percentage area treated weighted by the number of years since treatment. We determined that the Treatment Index for unoccupied habitat was more than four times that of occupied habitat, indicating that vireos selected habitat that was less treated in which to settle. We monitored vireo nests at three general site types: (1) within the flood channel where exotic and native vegetation removal has occurred regularly (Channel), (2) three sites near the flood channel where limited exotic and native vegetation removal has occurred (Off-channel), and (3) three sites that have been actively restored by planting native vegetation (Restoration). Nesting activity was monitored in 80 territories, 3 of which were occupied by single males and 1 by a male whose breeding status could not be confirmed. Overall, 38 percent of completed nests were successful and nest success did not differ among the three sites. In 2022, there were no differences with regard to clutch size, hatching, or fledging success among Channel, Off-channel and Restoration sites. Overall breeding success and productivity were

slightly higher in 2022 than in 2021, with 72 percent of pairs fledgling at least one young and pairs fledging an average of 2.2±1.7 young. To investigate if the cumulative years of treatment had an effect on vireo reproductive effort, we looked at the effects of the Treatment Index on reproductive parameters. Results from generalized linear models indicated that treatment did not have an effect on vireo nesting effort or the number of vireo fledglings per pair produced in 2022. Similarly, we did not detect an effect of Treatment Index on daily survival rate (DSR) of nests. Analysis of vegetation data collected at vireo nests from 2006 to 2022 did not reveal an effect of vegetation cover at the nest on DSR. We did find, however, that Channel nests were placed higher in the host plant than Off-channel nests. In the Channel and Off-channel sites, successful nests were placed closer to the edge of the host plants than unsuccessful nests. Additionally, successful Off-channel nests were placed lower in the vegetation, in shorter host plants, and closer to the edge of the vegetation clump than unsuccessful nests. Red/arroyo willow (Salix laevigata or Salix lasiolepis) were the species most commonly selected for nesting by vireos in all three site types. Black willow (Salix gooddingii) and mule fat (Baccharis salicifolia) also were commonly used. Vireos used a wider variety of species for nesting in Channel and Off-channel sites (eight and six species, respectively) compared to Restoration sites (two species), although there was limited nesting in Restoration sites in 2022. There were 43 vireos banded before the 2022 breeding season that were resighted and identified at the Project Area in 2022, all of which were originally banded in the Project Area. Adult birds of known age ranged from 1 to 7 years old. A total of 146 vireos were newly banded in 2022. There were 8 adult vireos banded with a unique color combination, and 138 nestlings were banded with a single dark blue numbered federal band on the left leg. Between 2006 and 2022, survivorship of males (66±11 percent) was consistently higher than that of females (59±12 percent). First-year birds from 2006 to 2022 had an average annual survivorship of 15±6 percent.

2 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report First-year dispersal in 2022 averaged 6.7±7.4 kilometers (km), with the longest dispersal (15.3 km) by a male that was recaptured at Fallbrook Creek, Fallbrook Naval Weapons Station (FNWS). From 2007 to 2011, most returning first-year vireos returned to the Project Area, whereas from 2014 to 2016, the majority of returning birds dispersed to areas outside of the Project Area. From 2018 to 2021, the trend shifted, and more first-year vireos returned to the Project area. In 2022, only one first-year vireo returned to the project area and two dispersed to sites outside the Project Area (upstream to the middle San Luis Rey River and to Fallbrook Creek, FNWS). However, the total number of identified first-year vireos was low and the trend in 2022 will likely shift as additional returning first-year vireos are identified in subsequent years. Most of the returning adult male vireos showed strong between-year site fidelity to their previous territories. Seventy-three percent of males (27/37) occupied a territory in 2022 that they had defended in 2021 (within 100 meters [m]). There were no females (0/4) detected in 2022 that returned to a territory they occupied in 2021; however, 50 percent of females (2/4) detected in 2022 returned to areas adjacent to their previous territories (within 300 m). The average between-year movement for returning adult vireos was 0.3±0.7 km. The amount of treatment at adults’ 2021 territories did not affect the distance adults moved to their 2022 territories. We completed four protocol surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus; flycatcher) at the Project Area between May 16 and July 25, 2022. Four transient Willow Flycatchers were detected in the Project Area in 2022. Two transients were detected in Reach 1, one in Reach 3a, and one in Pilgrim Pond. There were not any resident flycatchers documented in the Project Area in 2022. A total of 46 vegetation transects (528 points) were sampled at the Project Area in 2022. Seventy-one percent (378/528) of points were located in the Channel, and 22 percent (115/528) were in Upper Pond. The remaining 7 percent (35/528) of points were at the Whelan Restoration site. Foliage cover below 2 m was higher at the Channel points compared to Upper Pond and Whelan Restoration, which can be attributed to the dense herbaceous vegetation that grows after mowing. Above 2 m, foliage cover was similar at the Channel and Whelan Restoration sites and was higher than at Upper Pond. Average canopy height was higher in the Channel (5.6±3.4 m) compared to Upper Pond (4.7±2.9 m) and Whelan Restoration (4.6±1.9 m). From 2006 to 2022, total foliage cover declined above 2 m in the Channel, in contrast to Upper Pond and Whelan Restoration, where little directional change in vegetation cover has occurred and where vegetation cover has largely recovered to 2006 levels. Within the Channel, the steepest declines occurred between 2009 and 2013 and between 2014 and 2016. Since 2016, we observed an increase in foliage cover, largely herbaceous, between 0 and 2 m within the Channel. The percent cover remained below levels detected before 2009 for other height classes.

We sampled vegetation at 44 vireo nests and 44 random plots (“territory” plots) within territories in the Channel and Upper Pond after the 2022 breeding season. Vireos in the Channel established territories in areas with significantly more cover from 3 to 6 m but less cover below 2 m relative to the available habitat. Within territories, Channel vireos selected nest sites with significantly more foliage cover from 2 to 3 m. Vireos at Upper Pond established territories in areas with significantly more foliage cover from 5 to 6 m and below 1 m relative to available habitat. However, within territories, Upper Pond vireos selected nest sites with significantly less foliage cover from 5 to 6 m and below 1 m. Data either are not available or have limited availability owing to restrictions of the funding entity (U.S. Army Corps of Engineers). Please contact Christopher Chabot, Planning Division, Los Angeles District, U.S. Army Corps of Engineers, for more information.

Introduction Least Bell’s Vireo The Least Bell’s Vireo (Vireo bellii pusillus; vireo) is a small, migratory, songbird that breeds in southern California and northwestern Baja California, Mexico, from April through July (Kus and others, 2020). Historically abundant within lowland riparian ecosystems, vireo populations began declining in the late 1900s as a result of habitat loss and alteration associated with urbanization and conversion of land adjacent to rivers and agriculture (Franzreb, 1989; U.S. Fish and Wildlife Service, 1998; Riparian Habitat Joint Venture, 2004). Additional factors contributing to the vireo's decline have been the expansion in range of the brood-parasitic Brown-headed Cowbird (Molothrus ater; cowbird), to include the Pacific Coast (U.S. Fish and Wildlife Service, 1986; Franzreb, 1989; Kus, 1998, 1999; Kus and others, 2020), and the introduction of invasive exotic plant species such as giant reed (Arundo donax) into riparian systems. By 1986, the vireo population in California numbered just 300 territorial males (U.S. Fish and Wildlife Service, 1986). In response to the dramatic reduction in numbers of vireos in California, the California Fish and Game Commission listed the species as endangered in 1980, with the U.S. Fish and Wildlife Service (USFWS) following suit in 1986. Since listing, the vireo population in southern California has rebounded, which is largely in response to cowbird control and habitat restoration and preservation (Kus, 1999; Kus and Whitfield, 2005). As of 2006, the statewide vireo population was estimated to be approximately 2,500–3,000 territories (U.S. Fish and Wildlife Service, 2006a), of which approximately 10 percent were along the San Luis Rey River between Interstate 15 and Interstate 5.

Introduction 3 Male vireos arrive on breeding grounds in southern California in mid-March. Male vireos are vocally conspicuous and frequently sing their diagnostic primary song from exposed perches throughout the breeding season. Females arrive approximately 1–2 weeks after males and are more secretive. Often, females are seen early in the season traveling through habitat with the male. The female, with the male's help, builds an open cup nest in dense vegetation approximately 1 meter (m) above the ground. Clutch size for vireos averages three to four eggs. Typically, the female and male incubate the eggs for 14 days and young fledge from the nest at 11–12 days of age. It is not unusual for vireos to re-nest after a failed attempt provided ample time remains within the breeding season. Vireos rarely fledge more than one brood in a season, although double-brooding can be more common during years when breeding conditions are favorable (early initiation, high early fledging success; Ferree and Kus, 2008b; Houston and others, 2017, 2019). Nesting lasts from early April through July, but adults and juvenile birds remain on the breeding grounds into late September through early October before migrating to their wintering grounds in southern Baja California, Mexico (Kus and others, 2020).

Southwestern Willow Flycatcher The Southwestern Willow Flycatcher (Empidonax traillii extimus; flycatcher) is one of four subspecies of Willow Flycatcher in the United States, with a breeding range that includes southern California, Arizona, New Mexico, extreme southern parts of Nevada, Utah, and Colorado, and western Texas (Unitt, 1987; U.S. Fish and Wildlife Service, 2002; Sedgwick, 2020). Restricted to riparian habitat for breeding, the flycatcher has declined in recent decades in response to widespread habitat loss throughout its range and, possibly, brood-parasitism by cowbirds (Remsen, 1978; Unitt, 1987; Schlorff, 1990; Whitfield and Sogge, 1999; U.S. Fish and Wildlife Service, 2002; Sedgwick, 2020). By 1993, the species was believed to number approximately 70 pairs in California (U.S. Fish and Wildlife Service, 1993), in small, disjunct populations. The flycatcher was listed as endangered by the State of California in 1992 and by the USFWS in 1995. Flycatchers in southern California co-occur with vireos; however, unlike the vireo, which has increased tenfold since the mid-1980s in response to management alleviating these threats (U.S. Fish and Wildlife Service, 2006a), flycatcher numbers have remained low. Currently (2022), most flycatchers in California are concentrated at one site: the upper San Luis Rey River at Lake Henshaw in San Diego County (Howell and Kus, 2022). Outside of this site, flycatchers occur as small, isolated populations of one to six pairs.

Male flycatchers typically arrive in southern California in early to mid-May, whereas females arrive approximately 1 week later. While on the breeding grounds, males sing repeatedly from exposed perches. Once the pair bond is established, the female builds an open cup nest that usually is placed in a branch fork of a willow (Salix spp.) or plant with a similar branching structure approximately 1–3 m above the ground. The typical clutch of three to four eggs is laid in May–June. Females incubate for approximately 12 days, and nestlings fledge within 12–15 days, in early July. Adults usually depart from their breeding territory in mid-August through early September to their wintering grounds in central America and northern South America (U.S. Fish and Wildlife Service, 2002).

San Luis Rey Flood Risk Management Project Area The San Luis Rey Flood Risk Management Project Area (Project Area) spans approximately 233 hectares (ha; 576 acres) of the lower San Luis Rey River in northwestern San Diego County, California (fig. 1; table 1). Authorized in 1970 and constructed during the late 1980s and early 1990s, the flood control Project Area includes single- and double-levee reaches and six off-channel detention ponds, five of which also serve as mitigation sites for effects to biological resources within the channel. Operation and maintenance of the flood control project within the channel includes periodic vegetation clearing, exotic plant removal, and sediment removal to ensure that sufficient water conveyance capacity is maintained (U.S. Fish and Wildlife Service, 2006b). Management of the off-channel ponds involves adaptive habitat management, such as vegetation clearing, exotic plant removal, and planting native vegetation. Riparian vegetation communities at the Project Area include willow-dominated riparian, mixed mule fat (Baccharis salicifolia) and sandbar willow (Salix exigua) riparian scrub, freshwater marsh, and areas dominated by non-native giant reed. Dominant plants include red/arroyo willow (Salix laevigata/Salix lasiolepis; we did not distinguish between these species), black willow (Salix gooddingii), Fremont cottonwood (Populus fremontii), sandbar willow, mule fat, and giant reed. Adjacent habitat and land-use types include coastal sage scrub, non-native grassland, and urban housing and commercial developments. Human disturbances such as transient camps, recreation, illegal dumping, introduction of invasive exotic plants and feral animals, and use by pets from neighboring houses are pervasive throughout the Project Area (table 2).

4 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report 117°23'

117°22'

117°21'

117°20'

117°19'

117°18'

SAN DIEGO COUNTY 33° 15'

ui s R ey R iv

San L

33° 14'

San Luis Rey Flood Risk Management Project Area 33° 13'

Off-channel

Channel Reach 1 Reach 2 Reach 3a Reach 3b Reach 4

33° 12'

750

Restoration

Lower Pond Park Pond Pilgrim Pond Riverside Pond Tuley Canyon Upper Pond Whelan Mitigation

Benet College Whelan

Meters 3,000

1,500 Western Ecological Research Center San Diego Field Station

Figure 1. Least Bell’s Vireo and Southwestern Willow Flycatcher survey and monitoring sites at the San Luis Rey Flood Risk Management Project Area, California, in 2022. Table 1. Least Bell’s Vireo and Southwestern Willow Flycatcher survey sites at the San Luis Rey Flood Risk Management Project Area, California, in 2022. Survey site

Description

Survey site

Channel sites Reach 1

From Interstate 5 to Benet Road.

Benet Restoration

Active restoration begun in 2012.

Reach 2

From Benet Road to Foussat Road.

Reach 3a

From Foussat Road to Whelan canal.

Reach 3b

From Whelan canal to Douglas Drive.

Reach 4

From Douglas Drive to College Boulevard.

College Restoration Active restoration begun in 2014.

Description Off-channel sites—Continued

Pilgrim Pond

Detention pond with historic restored habitat, north of Reach 4.

Riverside Pond

Detention pond with historic restored habitat, north of Reach 3b.

Tuley Canyon

Detention pond with historic restored habitat, west of Benet Road and north of the levee.

Upper Pond

Detention pond with historic restored habitat located south of levee between College Boulevard and Douglas Drive. Adaptive habitat management (mowing in 2015 and planting in 2017).

Whelan Mitigation

Historic restored habitat north of levee, between Whelan canal and Foussat Road.

Off-channel sites Lower Pond

Detention pond with historic restored habitat, west of Benet Road and south of the levee.

Park Pond

Detention pond with historic restored habitat, located south of the levee, between Douglas Drive and Foussat Road. Adaptive habitat management (mowing in 2015 and planting in 2017).

Whelan Restoration Active restoration begun in 2014.

Mixed willow

Red/arroyo willow, black willow

Exotic herbaceous cover

Transient camps; moderate-heavy human use

Dominant canopy species

Dominant exotic species

Disturbance

1Listed in order of dominance.

148

Habitat type1

Channel

Size: hectares

Attribute

Transient camps, pets, recreation; heavy human use

Exotic herbaceous cover

Sandbar willow, mule fat

Riparian scrub

Park Pond

Upper Pond

Off-channel

Transient camps, pets, recreation; heavy human use

Exotic herbaceous cover

Sandbar willow, mule fat, red/arroyo willow, black willow

Riparian scrub/mixed willow

[Channel: Reach 1, Reach 2, Reach 3a, Reach 3b, and Reach 4 survey sites]

Transient camps; moderate-heavy human use

Exotic herbaceous cover

Black willow, mule fat, red/arroyo willow

Mixed willow/riparian scrub

Whelan Mitigation Mixed willow/riparian scrub

10.5

Whelan Restoration

Exotic plant control; moderate transient use

Exotic herbaceous cover

Exotic plant control; some transient use

Exotic herbaceous cover

Red/arroyo willow, Sandbar willow, mule black willow fat, red/arroyo willow, black willow

Mixed willow

Benet Restoration

Restoration

Exotic plant control; some transient use

Exotic herbaceous cover

Black willow, mule fat, red/ arroyo willow

Mixed willow

0.4

College Restoration

Table 2. Site attributes of the Channel, Off-channel, and Restoration monitoring sites at the San Luis Rey Flood Risk Management Project Area, California, in 2022.

Introduction 5

6 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report The Project Area includes a channelized 10.7-kilometer (km) section of the lower San Luis Rey River from Interstate 5 to College Boulevard and six detention ponds outside of the channel in Oceanside, California (table 1). The Project Area is divided into 12 survey sites: 5 of which are primarily within the flood control channel (hereafter “Channel”) and 7 that are outside of the channel (hereafter “Off-channel”; table 1). The seven Off-channel sites include one historic restored site north of the levee and west of Whelan Lake and six in the detention ponds. There are two large restoration sites: one in Reach 1 (Benet Restoration) and one in Whelan Mitigation (Whelan Restoration). One smaller restoration site is in Reach 4 (College Restoration). Before the 2006 vireo breeding season, the U.S. Army Corps of Engineers began two flood risk management activities for the San Luis Rey Flood Risk Management Project: (1) exotic species eradication and (2) flood risk management of river channel vegetation. Exotic species eradication primarily included removal and control of giant reed, but other exotic species such as pepperweed (Lepidium latifolium), tamarisk (Tamarix ramosissima), and other non-native tree species also were targeted for removal. Exotic species were physically removed or treated with herbicide. The purpose of the flood risk management was to remove vegetation from the San Luis Rey flood control channel to provide a level of flood conveyance of 2,016 cubic meters per second (m3/s; equivalent to a 150-year flood; U.S. Fish and Wildlife Service, 2006b). In addition, a one-time mowing and mulching task was performed in 2005 to reduce the risk of flooding during the 2006 rainy season. On-going flood risk management of river channel vegetation includes rotational and annual vegetation mowing in phases. Phase 1 includes vegetation clearing in Reaches 1–4 to achieve a minimum flow conveyance of 1,500 m3/s (equivalent to a 100-year flood). Phase 2 expanded the area mowed beyond that of Phase 1 and included creation of habitat for vireos and flycatchers. Phase 3 includes mowing associated with maintaining the Phase 1 mowing area as well as small sections of vegetation mowing in Reach 1 and Reach 3a. Rotational mowing is done every 5 years, alternating between two 18–23-m-wide strips of vegetation (rotations 1 and 2) such that each strip is mowed every 10 years. Prior to the 2022 vireo breeding season, there was mowing that occurred in the river channel. In fall 2021, annual Phase 1 and 2 mowing in Reaches 1–4 was completed as part of the overall Flood Risk Management Project efforts. For a detailed description of the timeline and vegetation treatments and management see appendix 1. There were three Restoration areas within the Project Area: an exotic plant removal program in Reach 1 (Benet Restoration) that began in October 2012 and two projects that started in March 2014 (appendix 2, figs. 2.1–2.4). The largest project, focused on providing flycatcher habitat, was

on the north side of the river channel in the eastern section of the Whelan Mitigation site (Whelan Restoration). A rock levee was removed and graded to below ground level, allowing flooding from the main channel to mimic more natural river flows. A second, smaller project occurred on the south side of Reach 4 near the College Boulevard bridge (College Restoration). Restoration activities included planting, bi-monthly watering, and spraying weeds with herbicide through 2015. Since 2016, there has been no watering or herbicide treatment, but we continued to classify these areas as Restoration sites and evaluate them separately. In addition to the three Restoration sites, in spring 2014, several small areas of the river were planted with pole cuttings, and in 2017, two of the Off-channel ponds were planted as a part of adaptive habitat management. These areas were considered passive restoration and were not included with our Restoration sites or in any of our analyses of Treatment Index.

Purpose and Scope The purpose of this study was to document the status of vireos and flycatchers at the Project Area and characterize habitat structure and composition within the Project Area. Specifically, our goals for vireos were to (1) determine the size and composition of the vireo population; (2) characterize habitat used by vireos; (3) band vireos and resight banded vireos to estimate vireo survivorship, site fidelity, and dispersal; and (4) assess the effects of vegetation removal on vireo reproductive success and productivity by monitoring established nest plots in the Channel and Off-channel sites. Our goals for flycatchers were to (1) determine the size and composition of the Willow Flycatcher population at the Project Area, (2) document and monitor nesting activities of resident flycatchers, and (3) band and resight all flycatchers to facilitate the estimation of flycatcher survivorship and movement. The purpose of the habitat component of the study was to (1) provide post-treatment data on the habitat composition and structure of the Project Area, including areas that underwent vegetation removal in 2005, 2007–12 and 2014–15, 2017–19, and 2021 and (2) characterize habitat use by vireos in response to vegetation management at the Project Area. The USGS has been monitoring the vireo population in the Project Area since 2006. These data, when combined with data from other years, will inform natural resource managers about the status of these endangered species at the Project Area and guide modification of land-use and management practices as appropriate to ensure the species’ persistence. Surveys and monitoring for the endangered Least Bell’s Vireo were performed at the San Luis Rey Flood Risk Management Project Area (Project Area) in the city of Oceanside, San Diego County, California, between April 11 and July 8, 2022.

Methods 7

Methods Surveys Vireo and flycatcher surveys have been performed at the Project Area every year since 2006. Four protocol vireo surveys were completed between April 11 and July 8, 2022. The surveys were done by following standard survey techniques developed and recommended by the California Least Bell’s Vireo Working Group (now known as the California Riparian Birds Working Group) and USFWS Least Bell’s Vireo survey guidelines (U.S. Fish and Wildlife Service, unpub. data, 2001). We supplemented these protocol surveys with weekly territory monitoring visits; therefore, most survey sites were visited more than 10 times throughout the breeding season, resulting in complete coverage of the Project Area. During the surveys and monitoring, we used recorded callbacks (as needed) to confirm absence of vireos in areas in which no birds were detected and to attract males to check leg bands. Vireo field work was done by Alexandra Houston, Suellen Lynn, Jessica Medina, Shannon Mendia, Ryan Pottinger, Devin Taylor, and Stéphane Vernhet (all under USFWS permit ESPER0004080_0), with assistance provided by Rafal Banas and Ynez Diaz. We completed four protocol flycatcher surveys of the Project Area between May 16 and July 25, 2022, completing one survey during each of four survey periods (U.S. Fish and Wildlife Service, 2000, Sogge and others, 2010). Flycatcher field work was done by Aaron Gallagher, Alexandra Houston, Suellen Lynn, Shannon Mendia, and Ryan Pottinger (all under USFWS permit ESPER0004080_0), with assistance provided by Rafal Banas, Ynez Diaz, Devin Taylor, and Stéphane Vernhet. For both species, observers moved slowly (1–2 kilometers per hour) through the riparian habitat while searching and listening for vireos or flycatchers. Observers walked along the north and south levees to survey the flood control channel. In wider stands, observers traversed the habitat to detect all birds throughout its extent. Surveys were completed between dawn and early afternoon, depending on wind and weather conditions. For each bird encountered, investigators recorded age (adult or juvenile), sex, breeding status (paired, single, unknown, or transient), and if the bird was banded. Birds were considered transients if they were not detected on two or more consecutive surveys after an initial detection. Vireo locations were mapped using Environmental Systems Research Institute (Esri) Field Maps (Environmental Systems Research Institute, 2022a) and Survey 123 (Environmental Systems Research Institute, 2022b) on Samsung Galaxy S7 and S8 and LG G5 mobile phones with Android operating systems and built in Global Positioning

System (GPS) to determine geographic coordinates (World Geographic System of 1984, WGS 84). Distance to the nearest surface water was recorded for each flycatcher location. Dominant native and exotic plants were recorded within each vireo and flycatcher territory, and percent cover of native vegetation was estimated using cover categories of less than 5 percent, 5–50 percent, 51–95 percent, and greater than 95 percent. Overall habitat type was specified according to the following categories: Mixed willow riparian: Habitat dominated by one or more willow species, including black willow, arroyo willow, and red willow, with mule fat as a frequent co-dominant. Willow-cottonwood: Willow riparian habitat in which cottonwood is a co-dominant. Willow-sycamore: Willow riparian habitat in which California sycamore (Platanus racemosa) is a co-dominant. Sycamore-oak: Woodlands in which California sycamore and coastal live oak (Quercus agrifolia) occur as co-dominants. Riparian scrub: Dry or sandy habitat dominated by sandbar willow or mule fat, with a few other woody species. Upland scrub: Coastal sage scrub adjacent to riparian habitat. Non-native: Areas vegetated exclusively with non-native species, such as giant reed and tamarisk.

Nest Monitoring We monitored vireo nests to evaluate the effects of native and exotic vegetation removal on nest success and productivity. Nest monitoring was performed from April 4 until the last nest fledged on July 31, 2022. We performed work at three general site types: (1) within the flood channel where exotic and native vegetation removal has occurred regularly (Channel); (2) at three sites next to the flood channel where limited exotic and native vegetation removal has occurred (Upper Pond, Park Pond, and Whelan Mitigation; Off-channel); and (3) at three sites that have been restored (Restoration), two of which were within the Channel (Benet Restoration in Reach 1 and College Restoration in Reach 4) and one situated Off-channel (Whelan Restoration within Whelan Mitigation).

8 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Territory boundaries were delineated by biologists in the field by circumscribing vireos’ nesting, singing, and foraging locations. In 2017, we began excluding areas in a vireo’s territory that were obviously avoided (for example, the vireo used the edges of the river channel but avoided the center) by creating two unconnected polygons, as opposed to a single large one spanning the area. We classified territories quantitatively by site type in ArcGIS (version 10.8.1; Environmental Systems Research Institute, 2020) using territory boundaries and the boundaries of the three site types (Channel, Off-channel, and Restoration). If a territory straddled more than one site type, we assigned it to the site type that comprised more than 50 percent of the territory. There were 61 Channel territories (60 pairs and 1 single male), 16 Off-channel territories (14 pairs, 1 single male, and 1 male whose breeding status could not be confirmed), and 3 Restoration territories (2 pairs and 1 single male) that were monitored during the 2022 breeding season. Pairs were observed for evidence of nesting, and their nests were located. We visited nests as infrequently as possible to minimize the chances of leading predators or cowbirds to nests; typically, there were three to four visits per nest. The first visit was timed to determine the number of eggs laid, the next visits to determine hatching and age of young, and the last to band nestlings (see the “Banding” section). We removed cowbird eggs from nests depending on when they were found. In nests with fewer than three vireo eggs, cowbird eggs were removed no sooner than the seventh day of incubation to minimize the possibility of nest abandonment in response to the removal. We removed cowbird eggs from nests that contained three or more vireo eggs as they were detected. Cowbird nestlings were removed immediately from nests. Fledging was determined through direct observation of fledglings in the territory or, in some rare cases, inferred from an accumulation of feather dust and fecal material in the nest, which is indicative of vireo fledging. We recorded characteristics of nests, including nest height, host plant species, host height, distance to edge of host plant, and distance to edge of host clump after abandonment or fledging of nests.

Analysis of Nesting Data We used Pearson’s chi-square analysis and Fisher’s Exact tests to determine if there were differences in nest success, hatching rates, and fledging rates among Channel, Off-channel, and Restoration pairs. We included a “Mixed” category for instances where (1) a territory was categorized as one site type, but the vireos placed at least one nest in another site type or (2) vireos nested in two different site types within the same territory. For example, we had territories that were categorized as “Restoration” (50 percent or more of the total territory area fell within the boundaries of the Restoration), but the vireos placed their nest(s) within the Channel, avoiding areas of active restoration. In total, four territories were considered “Mixed” in 2022. For analyses involving the nest as the unit of analysis, we used the location of the nest (for example, a territory categorized as Channel could have a nest in a Restoration site). For any analyses of reproductive success or productivity that involved a measure of success per pair or territory, we analyzed Mixed territories separately. Chi-square tests were used when sample sizes were sufficient; Fisher’s Exact tests were used when one or more categories contained fewer than five samples. We used two-sample t-tests, Mann-Whitney U-tests (two groups), and Kruskal-Wallis tests (three or more groups) to determine if there were differences in nest site characteristics between successful and unsuccessful nests within and among the Channel, Off-channel, and Restoration sites. T-tests were used when distributions were normal and variances were similar; Mann-Whitney U-tests or Kruskal-Wallis tests were used when the data violated these assumptions. We used Analysis of Variance and Tukey’s post-hoc pair-wise comparisons to determine if there were differences among Channel, Off-channel, and Restoration sites in average clutch size and average number of young per pair in 2022 and average clutch size and number of young fledged per pair by year (2006–22).

Methods 9

Effects of Treatment We created an index of treatment (Treatment Index) to evaluate the cumulative effects of vegetation removal over time for 2021 and 2022 vireo territories. We restricted treatment type for this index to the vegetation removal that occurred from 2005, 2007–12, 2014–15, 2017–19 or 2021, in which vegetation (native or exotic) was cleared to the ground. We did not include any restoration or herbicide treatments because these treatments were distinct from vegetation removal and analysis of them was beyond the scope of this project. We limited our analysis of Treatment Index to territories in the Channel because Off-channel adaptive habitat management activities mitigated the effect of the vegetation removal, making the Treatment Index for those territories unsuitable for our analyses. We calculated the percentage of each territory area that was treated in each year (2005, 2007–12, 2014–15, 2017–19, and 2021) by overlaying the 2021 and 2022 territory boundaries with the treatment areas in ArcGIS (version 10.8.1; Environmental Systems Research Institute, 2020). Territory boundaries were created to include the bird location points collected using GPS throughout the field season. Treatment boundaries were generated from the location data collected using GPS in the field by a RECON Environmental, Inc. (https://www.recon-us.com/) biologist who walked the treatment boundary during vegetation removal activities. Because treatments have been occurring since 2005, it was possible to have multiple years of treatments as well as overlap among treatments within a single territory. Because early successional vegetation can recover quickly after disturbance, we assumed that the influence of the vegetation removal diminished over time; therefore, when there was overlap in treated areas among years, we used the most recent treatment year to calculate the Treatment Index (fig. 2). To calculate the Treatment Index, we used the percentage of the territory area treated weighted by the squared inverse of the time (t2) since treatment summed across all treatment years. We chose t2, rather than t because we wanted to capture the effect of the treatment diminishing quickly over time. We used the following formula: 2021

Treatment Index = ∑  p ercent Territory Area Treated  t × 1 / n t2  t=2005

where

t n n=1

is the year of treatment, is the years since treatment, and is the first breeding season after treatment.

2005 Treatment

Untreated

2014 Territory

Overlap in 2005 and 2008 treatments

*Use 2008, the most recent treatment year

2008 Treatment

Figure 2. Least Bell’s Vireo territory boundary with two treatments (2005 and 2008 vegetation removal) and a section of overlap between the two treatments at the San Luis Rey Flood Risk Management Project Area, California, 2022.

(1)

Note that there was no treatment in 2006, 2013, 2016 or 2020, so values for these years = 0 and drop out of the equation. We performed three sets of analyses using the Treatment Index. For all analyses of the Treatment Index, we excluded territories that overlapped with areas of active restoration: 1. We compared the Treatment Index between occupied (monitored and non-monitored) territories and unoccupied vireo habitat within the Channel using a two-sample t-test. To generate a Treatment Index for unoccupied habitat, we used historic territory boundaries from 2006 to 2021 as a guide and sub-divided habitat that was unoccupied in 2022 into units approximating vireo territories in size. We calculated the Treatment Index for the unoccupied historical territories within the Channel that did not overlap with restoration, creating a distribution of Treatment Indices for unoccupied habitat that allowed statistical comparisons with occupied territories. Habitat never occupied by vireos (for example, marsh, open water) during this study and unoccupied patches too small (less than or equal to 0.2 ha [0.5 acres]) for a territory were excluded from analysis. We hypothesized that the Treatment Index in occupied habitat would be lower than that of the unoccupied habitat, indicating that vireos were preferentially selecting habitat that was less treated or was treated less recently.

10 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report 2. We used generalized linear models (GLMs) in Program R (R Core Team, 2021) to evaluate if treatment (Treatment Index) had an effect on vireo reproductive parameters for monitored territories within the Channel, including the total number of (1) nests, (2) completed nests, and (3) fledglings per territory in 2022. We limited our analysis to Channel territories since 2018 because passive restoration, including some planting, weeding, and watering, occurred at two of our Off-channel sites in 2017, confounding the effect of the vegetation removal. We hypothesized that there would be an inverse relationship between the reproductive parameters and the Treatment Index (for example, the number of fledglings would decrease as the Treatment Index increased) and a positive relationship between the Treatment Index and the number of nests, as we might expect to see higher levels of nest failure in the lower quality (more treated) habitat. Treatment Index was a continuous, fixed effect. We assumed a Poisson distributed response to the reproductive parameters. 3. Finally, we used a GLM to test for the effect of the expected Treatment Index for 2021 territories (that is, the Treatment Index in 2022 if the bird had occupied exactly the same territory boundaries as 2021) on the distances that banded vireos moved between 2021 and 2022. We hypothesized that birds whose previous territories were heavily treated (had high Treatment Indices) would shift their territories a greater distance than those whose territories were less affected by, or were temporally further removed from, the treatment.

Nest Survival Analysis We used RMark (Laake, 2013) to model the effects of vegetation treatment on daily survival rate (DSR) of vireo nests (or the probability that a nest survived from 1 day to the next; Dinsmore and others, 2002). We used RMark (Laake, 2013) to run program MARK (White and Burnham, 1999) to calculate DSR, which accounts for the variability in exposure days across nests detected at different stages of the nesting cycle and allows for the analysis of the effects of covariates on DSR. In this study, we evaluated the effects of Treatment Index (for Channel territories only) on DSR in 2022. We studied habitat structure at the nest (2006–22) to determine if it was correlated with DSR by analyzing the relationship between total vegetation cover at each height category (see the “Vegetation Study Design” section) and DSR. We calculated nest survival across a 32-day cycle (2 days to complete the nest and rest before the first egg is laid, 4 days egg-laying, 14 days incubation, 12 days nestling period), in which incubation begins with the penultimate egg.

We calculated age of nests at the time they were discovered by forward- or backward-dating of nests in relation to known dates of nest building, laying, or hatching. We used an information-theoretic approach (Burnham and Anderson, 2002) to evaluate support for statistical models reflecting a priori hypotheses regarding the effect of treatment and habitat variables on DSR. We hypothesized that DSR would be inversely related to the Treatment Index and would increase with increasing foliage cover at nest sites, particularly understory cover within 2 m of the ground, where vireos place their nests. We used logistic regression with a logit link to build models. First, we generated a constant survival model to serve as a reference for the effect of treatment and habitat variables on DSR. We then modeled the treatment and habitat covariates individually and evaluated support for each model in relation to the constant survival model and each other. To evaluate the effect of covariates within our top models, we calculated the odds ratio for each covariate (the odds that the covariate had an effect on DSR where “no effect” equals 1, negative effect is less than 1, positive effect is greater than 1). We then calculated the 95-percent confidence interval of the odds ratio to determine the likelihood that the effect was significant. Where the confidence interval did not include 1 (was greater than or less than 1), we concluded that we had 95-percent confidence that the covariate had an effect on DSR relative to the reference.

Banding The primary goals of banding vireos at the San Luis Rey Flood Risk Management Project Area were to (1) assess vireo site fidelity in response to vegetation management, (2) examine natal dispersal within and outside of the Project Area in response to vegetation management, and (3) understand how vegetation removal and alteration affected vireo demography. We banded nestlings from monitored nests at 6–7 days of age with a single, dark blue anodized numbered federal band on the left leg. We captured adult vireos in mist nets at monitoring sites as needed to clarify identity among neighbors and banded them with a unique combination of colored plastic and anodized metal federal bands, including an anodized dark blue band to designate the San Luis Rey River as the bird’s site of origin. Adults previously banded with a single numbered metal federal band as nestlings (natal birds) were target netted to determine their identity, and their original band was supplemented with other bands to generate a unique color combination. These data will supplement banding data currently being gathered by the U.S. Geological Survey (USGS) and other investigators regarding nearby vireo populations on the upper San Luis Rey River and the Santa Margarita River on Marine Corps Base Camp Pendleton (MCBCP).

Methods 11

Survivorship Estimates During surveys and nest monitoring activities, we attempted to resight all vireos to determine if they were banded, and if so, to confirm the vireos’ identity by reading the unique color band combination or by recapturing birds with single federal bands. We used resighting and recapture data to calculate annual survivorship or the fraction of all individuals present at the Project Area in 1 year that returned the following year. Imperfect detectability of banded individuals is typical of mark-recapture studies and occurs for various reasons (for example, females are more cryptic and may be missed on surveys, birds were detected as banded but their full color combinations [and thus identities] were not obtained, or birds with single federal bands were not recaptured, so their identities were not determined). To account for individuals that were present but not detected, we used RMark (Laake, 2013) which uses program MARK (White and Burnham, 1999) to model annual survivorship. Annual survivorship was calculated from 2006 to 2022 by creating an encounter history matrix of all individual vireos detected in the Project Area and if they were observed in each year from 2006 through 2022. Vireos were grouped by age (originally encountered as a juvenile [first-year; usually nestling but sometimes fledglings] versus originally encountered as an adult) and sex (female versus male). Survivorship was assumed to be constant for adults once they survived their first year. We created two sets of models. The first set included only adult captures, in which we modeled the influence of sex, year and bio-year (July 1–June 30), and precipitation (National Oceanic and Atmospheric Administration, 2022) on vireo survivorship and the influence of sex on detection probability. For precipitation, we used precipitation data from the previous bio-year. For example, to model survival from 2021 to 2022, we used bio-year precipitation data from July 2020 through June 2021. The second model set included both adult and first-year vireos and modeled the influence of age, year, and precipitation on survivorship. This model set did not include sex because we were unable to determine sex of vireos banded as juveniles unless they returned and were recaptured and identified as adults. Therefore, only the juveniles that survived their first

winter were retroactively classified as “male” or “female,” which would severely bias the estimate of sex-related survivorship of first-year vireos. Models created for survivorship in RMark (Laake, 2013) included only detections from sites at which survey effort has been consistent from 2006 through 2022 (for example, lower San Luis Rey River Project Area, middle San Luis Rey River, and MCBCP core survey areas). Incidental resights outside of these survey sites were excluded from analysis. We excluded one adult of unknown sex from our first model-set analysis because we were not interested in defining characteristics of this group. We used an information-theoretic approach (Akaike’s Information Criterion for small sample sizes [AICc]; Burnham and Anderson, 2002) to evaluate support for the models. To evaluate the influence of sex and year on detection probability, we compared models holding survivorship constant. For the adults-only models, we determined that detection probability was influenced by sex, so it was included in all models of vireo survivorship. For models with adults and first-year vireos, we determined that detection probability varied by year. However, because detection probability and survival are confounded for the final time interval (in this case, the current year) in fully time-dependent models (with year affecting survival and detection probability), we modeled detection probability as a constant to get a more reliable estimate of survival for the most recent time interval. We compared estimates of annual survival from 2006 to 2021 generated by models where detection probability varied by year with those where detection probability was constant and determined that the survival estimates were very similar, yielding the same long-term means. We concluded that the similarity in estimates justified using the simpler model because it would yield a more reliable estimate of survival for the 2021–22 time interval. We used logistic regression with a logit link to build and rank models by AICc and present annual real estimates from the top model. If there was support for multiple models (change in AICc [ΔAICc] less than 2), we averaged all models using AICc weights to obtain annual real estimates of survivorship for adult females, adult males, all adults, and all first-year vireos. We also evaluated the effect of covariates within our top models by calculating the odds ratio for each covariate (see the “Nest Survival Analysis” section).

12 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Site Fidelity and Movement We determined site fidelity of adult banded vireos by measuring the distance between the center of an individual’s breeding territory in 2021 and the center of the same individual’s breeding territory in 2022. We considered vireos to have exhibited site fidelity if they returned to within 100 m of their 2021 territory. Site fidelity was calculated for the same categories analyzed for survivorship (see previous section), except that only individuals with known territory locations during the last year they were detected before 2022 were included (for example, juvenile birds banded after fledging were excluded because their natal territories could not be confirmed in light of their capacity for substantial movement). We examined site fidelity to identify patterns indicative of relaxation of site fidelity in response to habitat alteration associated with vegetation clearing. We analyzed the distance moved by territorial males, hypothesizing that movement would be greater (site fidelity weaker) in years after vegetation treatment than in years where vegetation was undisturbed. We investigated the effect of a Treatment Index (see the “Effects of Treatment” section) on the movement of birds between 2021 and 2022. We focused on banded males because sample sizes for banded females were low, and females, in general, tend to exhibit lower site fidelity than males. We examined first-year dispersal in 2022 and compared it to dispersal in previous years. We compared the proportion of first-year vireos that originated at the Project Area and returned to the Project Area to the proportion of first-year vireos that returned to nearby areas outside of the Project Area. Because some natal vireos were not detected or identified during their first year of dispersal but were recaptured in subsequent years, our initial estimates were adjusted to include vireos that were not detected during their first year of dispersal. Although we were unable to determine their exact territory location during their first year, we can conclude that birds recaptured outside of the Project site were not likely present at the Project Area because of our high success in recapturing and resighting vireos within the Project Area.

Vegetation Study Design We sampled vegetation along permanent linear transects within three of the vireo monitoring sites (Channel, Upper Pond and Whelan Restoration). Sampling points consisted of 2- by 2-m quadrats located at 10-m intervals along each transect; the number of points sampled varied with the length of each transect (fig. 3). Transects were originally established in 2006 using a systematic sampling design. To capture the range of variability of riparian vegetation structure and composition, we positioned transects perpendicular to the river channel. To provide uniform coverage, we placed transects at fixed distances from each other; distances varied with the size of the site. In the Channel, transects were placed at 200or 400-m intervals depending on the width of the river. In

Upper Pond, transects were placed every 100 m. In addition, we sampled two 350-m transects at Whelan Mitigation (Whelan Restoration) that were initially surveyed from 1991 to 1993 to monitor riparian restoration by the U.S. Army Corps of Engineers (Kus, 1998). The Whelan Restoration transects were 75 m apart and oriented approximately parallel (320 degrees) to the flood control channel. Before 2014, this site was sampled and used as an Off-channel site in analyses. However, in 2014, it became a Restoration site and has been analyzed separately from the Channel and the Upper Pond sites since then. We used several permanent and semi-permanent methods to ensure that transects could be re-sampled in future years. First, a 1.5-m metal rebar was driven into the ground, leaving 75 centimeters above ground to mark the start of each transect. We spray-painted the rebar pink or orange and placed them at the intersection of the south levee and the riverbed. From the rebar, using a compass and tape measure, two field personnel measured the distances between sampling points. A numbered wooden stake, spray-painted pink or orange, was placed in the ground and colored plastic flagging was tied nearby to aid in locating the points. Finally, we recorded geographic coordinates for each rebar and wooden stake using a GPS unit (Garmin GPS 12).

Vireo Habitat Use Study Design In addition to sampling vegetation along transects, we collected vegetation data at 44 randomly selected vireo territories. We sampled one nest and one paired random plot within each territory (hereafter “nest plots” and “territory plots,” respectively) in the Channel and at Upper Pond. We did not sample vegetation at Restoration site nests because the sample was small, and our focus was to determine the response of vireos to treatments (measured by the Treatment Index) rather than their response to restoration activities. Nest and territory plots consisted of four 2- by 2-m quadrats; one quadrat centered on the nest (or center for random plots) and the remaining three quadrats located 10 m from the nest/ center and oriented at 0, 120, and 240 degrees from it (fig. 4). Territory plot locations were constrained using the Buffer tool in ArcGIS (Environmental Systems Research Institute, 2020) to create a 10-m negative buffer on the territory boundaries. This negative buffer ensured that the random sampling point for the quadrats at 0, 120, or 240 degrees would not fall outside of the territory boundaries. A positive 2-m buffer was created around nest locations and vegetation transect points to exclude these locations and prevent duplicating vegetation sampling. We then used the “Create Random Points” tool on the territory boundary buffer to select one random point between the buffered areas to serve as the center quadrat. Territories were excluded from analysis if they were too narrow to allow a 10-m negative buffer, were in the “mixed” type category, or overlapped with Restoration.

Methods 13

Area Areaof ofvegetation vegetation mowing and and giant mowing giant need eradication need eradication

Sampling Point

Treated Point Treated Point

Untreated Point

Figure 3. Vegetation transects in the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Vegetation Sampling

Figure 4. Nest-centered and territory vegetation plots sampled at the San Luis Rey Flood Risk Management Project Area, California, 2022. Abbreviation: m, meter.

Foliage cover at 1-m height intervals was estimated using the “stacked cube” method, developed specifically to characterize canopy architecture in structurally diverse riparian habitat (Kus, 1998). At each sampling point along a vegetation transect or in a nest or territory plot, we recorded canopy height and percent cover of vegetation, by species, within 1-m-height intervals, using a modified Daubenmire (1959) scale with cover classes: less than 1, 1–10, 11–25, 26–50, 51–75, 76–90, and greater than 90 percent. The sampling units were 2- by 2- by 1-m-high cubes, which were stacked vertically between the ground and the top of the canopy. Four 2-m-length PVC pipes were placed on the ground to define quadrat boundaries, and a 7.5-m-tall fiberglass telescoping pole, demarcated in 1-m intervals, was used to determine height class and canopy height. Vegetation data were collected by USGS biologists Lisa Allen, Rafal Banas, Annabelle Bernabe, Ynez Diaz, Aaron Gallagher, Alexandra Houston, Scarlett Howell, Suellen Lynn, Jessica Medina, Shannon Mendia, Ryan Pottinger, Devin Taylor, Michelle Treadwell, and Stéphane Vernhet.

14 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Vegetation Data Analysis For analysis, we converted cover codes to class midpoints, which were then used to quantify foliage cover at each sampling point for nine height classes: 0–1 m, 1–2 m, 2–3 m, 3–4 m, 4–5 m, 5–6 m, 6–7 m, 7–8 m, and greater than 8 m. We examined percent cover for plant species that occurred at greater than 5 percent of the sampling points (more than 25 points). Species that were less common (less than 5 percent of the sampling points) were grouped together by plant life form, including tree, shrub, dead woody species, and freshwater marsh. Herbaceous species also were grouped together and included annual and perennial species (native and exotic), with the exception of nine exotic species: poison hemlock (Conium maculatum), pepperweed, fennel (Foeniculum vulgare), white sweet clover (Melilotus alba), black mustard (Brassica nigra), wild radish (Raphanus sativus), thistle (Cirsium sp.), ivy (Hedera sp.), and all grass species. These nine exotic species were combined and analyzed as exotics. We calculated average foliage cover across all height classes separately for the Channel, Upper Pond, and Whelan Restoration sites. We used linear regression to examine the cumulative effects of vegetation management activities at these sites over time (2006–22) and to investigate any temporal trends that existed in percent cover at all height classes for each site. We focused on a cumulative approach because it allowed us to analyze the long-term effects of vegetation management within the Project Area. Comparisons between the intensively managed Channel sites and the

less-managed Off-channel sites provided insight into how annual vegetation management has affected the structure of the vegetation in the Channel through time. We used data from the vegetation transects, nest plots, and territory plots to examine vireo habitat selection at two spatial scales. First, we compared vegetation structure at nest plots to that at territory plots to assess if vireos were selecting nest sites non-randomly, relative to the vegetation available in the territory. Next, we compared territory plot data to transect data to evaluate if vireos were establishing territories non-randomly with regard to the habitat available to them throughout the site. We performed these comparisons separately for the Channel and Upper Pond to evaluate the effect of vegetation treatment on vireo nest and territory placement. We used t-tests to test for differences in foliage cover between nest sites and the territory and between territories and the available habitat. Averages are presented with standard deviations. We considered P less than or equal to 0.10 to be significant for all statistical tests to avoid overlooking potentially important biological relationships relevant to the recovery of these endangered birds. Data were analyzed using Program R (R Core Team, 2021). Data from the Project Area from 2006 to 2021 used in comparisons with current data can be found in Ferree and Kus, 2007, 2008a, 2008b; Ferree and others, 2010a, 2010b, 2011, 2012, 2014, 2015; and Houston and others, 2015, 2016, 2017, 2018, 2019, 2021, 2022.

Results 15

Results

were in Benet Restoration, two in College Restoration, and the remaining 90 Channel territories were outside of Restoration areas. Of the Off-channel territories, 3 were within Whelan Restoration, 7 were within Whelan Mitigation but outside of the Restoration area, and 25 were in other retention ponds (tables 3, 4; appendix 2, figs. 2.1–2.4). From 2021 to 2022, the number of vireo territories in the Channel increased by 14 percent (from 86 to 98) and decreased by 3 percent (from 36 to 35) within Off-channel sites (table 4). Vireo territory numbers increased for all Channel sites except for Reach 2 and Reach 3b, which decreased by one and three territories respectively. The largest increase was in Reach 3a (six territories, 60-percent increase). In most of the Off-channel sites, the number of vireo territories did not change from 2021 to 2022; the exceptions were Lower Pond, which increased by one territory, and Upper pond, which decreased by two territories (table 4).

Least Bell’s Vireo Population Size and Distribution We identified a total of 133 vireo territories during surveys and weekly territory monitoring (table 3; appendix 2, figs. 2.1–2.4). The number of vireo territories increased from 122 in 2021, which is a 9 percent increase (table 4; fig. 5). Of the 133 territorial males, 114 (86 percent) were confirmed as paired and 3 (2 percent) were confirmed as single males. We were unable to confirm pair status for the remaining 16 territories. We detected two transient vireos during surveys in 2022 (table 3). Seventy-four percent of the territories (98/133) were within the Channel. The remaining 26 percent of the territories (35/133) were located Off-channel. Six Channel territories

Table 3. Number and breeding status of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022. Survey site

Known Unknown Single Total Total pairs status males territories transients Channel sites

Reach 1

Benet Restoration

Reach 2

Reach 3a

Reach 3b

Reach 4

College Restoration

Off-channel sites Lower Pond

Tuley Canyon

Whelan Mitigation

Whelan Restoration

Park Pond

Pilgrim Pond

Upper Pond

114

133

Riverside Pond Total

Reach 3b

Reach 4

Tuley Canyon

Whelan Mitigation

119

Park Pond

Pilgrim Pond

Upper Pond

Riverside Pond

Total

Grand total

—

Lower Pond

Whelan Restoration

Total

—

College Restoration

Reach 3a

—

2006

Reach 2

Benet Restoration

Reach 1

Survey Site

[—, no data]

—

108

—

2007

130

—

2008

171

—

118

—

2009

157

—

111

—

2010

130

—

2011

2013

—

103

—

Off-channel sites

—

Channel sites

2012

100

2014

2015

2016

2017

130

2018

Table 4. Number and distribution of Least Bell’s Vireo territories at the San Luis Rey Flood Risk Management Project Area, California, 2006–22.

128

2019

161

105

2020

122

2021

133

2022

16 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Results 17 180 Vireo territories

160

Total number of territories

140 120 100 80 60 40 20

19 84 19 85 19 86 19 87 19 88 19 89 19 90 19 91 19 92 19 93 19 94 19 95 19 96 19 97 19 98 19 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12 20 13 20 14 20 15 20 16 20 17 20 18 20 19 20 20 20 21 20 22

0 Year

Figure 5. Number of Least Bell’s Vireo territories from 1984 to 2022 at the San Luis Rey Flood Risk Management Project Area, California. Surveys were not conducted during 1985, 1988, 2001, or 2004.

18 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Effects of Treatment on Habitat Use To investigate if vireos were selecting territories in habitat that was less affected by vegetation removal, we compared the Treatment Index of occupied vireo territories in the Channel to the Treatment Index of territories previously occupied, but unused, in 2022. The average Treatment Index of unoccupied habitat (0.27±0.31, n=86) was 4.5 times that of habitat occupied by vireos in 2022 (0.06±0.10, n=77; t = −5.9, df=105, P<0.01), indicating that vireos avoided the more heavily treated sections of the river channel.

Nest Monitoring We monitored nesting activity in 80 territories within the San Luis Rey River Flood Risk Management Project Area monitoring areas (table 5; appendix 2, figs. 2.1–2.4). These territories were fully monitored, meaning that all nests within the territory were located and monitored during the breeding season. Of the 80 monitored territories, 3 were occupied by single males and 1 by a male whose breeding status could not be confirmed; therefore, these 4 territories were excluded from nesting analyses. A total of 161 nests were monitored during

the breeding season (appendix 3, table 3.1). Fifteen of these nests were not completed monitored and subsequently were excluded from calculations of nest success and productivity, unless otherwise noted.

Nesting Attempts In 2022, the average number of nesting attempts per territory (including incomplete nests) was 2.1±1.0 (table 5). A total of 72 percent (55/76) of pairs re-nested after their first nest attempt, and 38 percent (21/55) of these pairs initiated at least a third nesting attempt. A total of 11 pairs (22 percent, 11/51) attempted a re-nest after a successful nest; of these, 4 pairs (36 percent, 4/11) successfully fledged 2 broods. Most of the first nesting attempts in 2022 were initiated in mid- to late-April (fig. 6). The peak of nest initiation was during the weeks of April 10 and April 17 when 38 percent (29/76) of first nests were initiated. Nesting was earlier in 2022 (median Julian day 115; April 25) compared to 2021 (median Julian day 126; May 6) and was similar to the 16-year average (mean median Julian day 114.3 [April 24–25], 2006–21; fig. 7).

Table 5. Number of Least Bell’s Vireo territories and nests monitored at the San Luis Rey Flood Risk Management Project Area, California, 2022. [Mixed territories had a territory classification that differed from one or more nest classifications (two territories were classified as Off-channel but had a nest located in the Channel, one territory was classified as Channel, but nested Off-channel, and one territory was classified as Restoration, but had nests in the Channel). Incomplete nests were partially built, but not completed. False nests were partially built by the male only. Abbreviations: —, no data; ±, plus or minus]

Category Territories

Channel Off-channel Restoration 2

Mixed

Total

Total number of pairs

Total number of nests monitored

131

—

161

Incomplete nests

—

Total number of completed nests

119

—

146

Completed nests/pair

1.9±0.8

1.6±0.7

3.0±0.0

2.8±1.0 1.9±0.8

Total number of nest attempts/pair

2.1±1.0

1.8±0.8

3.0±0.0

3.0±0.8 2.1±1.0

1All nests were in Whelan Restoration.

Results 19

16 14

Number of nests

12 10 8 6 4 2 0

Week of nest initiation

Figure 6. Number of first Least Bell’s Vireo nests initiated (date first egg laid) by week at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Median Julian date of first nest initiation

130 125 120 115 110 105 100 95

2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022

Year

Figure 7. Annual median initiation dates of first nests (re-nests excluded) at the San Luis Rey Flood Risk Management Project Area, California, 2006–22.

20 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Apparent Nest Success Overall, 38 percent (55/146) of completed nests were successful (table 6). A total of 39 percent (46/119) of nests in the Channel, 38 percent (9/24) of Off-channel nests, and 0 percent (0/3) of Restoration nests fledged young. Nest success did not statistically differ among the three site types (P=0.57, Fisher’s Exact test). Causes of nest failure were similar among the Channel, Off-channel, and Restoration site nests. Predation was the primary source of nest failure for all sites (table 6). Predation accounted for 62 percent (45/73), 80 percent (12/15), and 67 percent (2/3) of nest failures in the Channel, Off-channel, and Restoration sites, respectively. Most predators were believed to be birds, mammals, or snakes. Overall, 38 percent (45/119) of Channel nests, 50 percent (12/24) of Off-channel nests, and 67 percent (2/3) of Restoration nests were lost to predation. Although we could attribute most nest failures to predation in 2022, there were 23 nests that failed for other or unknown reasons. Of these nests, 2 failed with infertile eggs, 1 was found after it had already failed, 1 was found with vireo and cowbird eggs under the nest, 1 failed because

a female vireo removed an egg, 2 were abandoned with eggs, and 16 nests failed for unknown reasons before eggs were documented.

Brown-headed Cowbird Parasitism We documented 40 instances of Brown-headed Cowbird parasitism in 2022, which is a 14-percent increase from 2021 (35 instances). In total, 40 nests (27 percent of completed nests) in 27 territories (36 percent of territories) were parasitized with 46 eggs, which is much higher than the 16-year average of 6 percent of completed nests parasitized (table 7; fig. 8). Nine nests failed as a result of parasitism (abandoned). We removed cowbird eggs from 30 nests, of which 10 (33 percent) were ultimately successful. Of the remaining parasitized nests, 19 were depredated, 1 failed after the removal of a cowbird egg, and 1 was found after it had failed with a broken vireo egg and cowbird egg below the nest. Parasitism was first observed during the third week of nesting (April 17), which is when 7 percent of completed nests were parasitized. The average weekly parasitism rate for completed nests from April 3 to July 3, 2022, was 31 percent (range 0–67 percent; fig. 9).

Table 6. Fate of completed Least Bell’s Vireo nests at the San Luis Rey Flood Risk Management Project Area, California, 2022. [Proportion of total completed nests shown in parentheses]

Nest fate

Number of nests Channel Off-channel Restoration

Successful

Total

55 (0.38)

Failed Predation

59 (0.40)

Parasitism

9 (0.06)

Other/unknown

23 (0.16)

Failed total

91 (0.62)

Total completed nests

119

146 (1.00)

Table 7. Number and fate of Least Bell’s Vireo nests parasitized by Brown-headed Cowbirds at the San Luis Rey Flood Risk Management Project Area, California, 2022. Category

Channel Off-channel Restoration Total

Nests parasitized

Total cowbird eggs laid

Fate of parasitized nests Abandoned

Not abandoned Successful

Unsuccessful

115

1One nest abandoned after the removal of cowbird egg; one nest found with vireo and cowbird eggs below nest.

Results 21 250

EXPLANATION Parasitized

200

Number of nests

Not parasitized 150

100

0 2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

2021

2022

Year

Figure 8. Number of completed Least Bell’s Vireo nests (by year) that were parasitized by Brown-headed Cowbirds at the San Luis Rey Flood Risk Management Project Area, California, 2006–22.

Number of nests

EXPLANATION

Parasitized Not parasitized

Apr 3

Apr 10 Apr 17 Apr 24

May 1

May 8 May 15 May 22 May 29

Jun 5

Jun 12 Jun 19 Jun 26

Jul 3

Week of nest initiation

Figure 9. Number of completed Least Bell’s Vireo nests initiated and those that were parasitized by Brown-headed Cowbirds (by week) at the San Luis Rey Flood Risk Management Project Area, California, 2022.

22 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Reproductive Success and Productivity Hatching and fledging success were similar in 2022 and 2021. Overall, 60 percent of nests with eggs hatched at least one egg, and 77 percent of nests with nestlings fledged at least one young (table 8), compared to 64 and 74 percent in 2021. Overall, clutch size of non-parasitized nests (3.2±0.5) was similar to 2021 (3.3±0.6) and the 16-year average (2006–21; 3.4). In 2022, there were no statistical differences among Channel, Off-channel and Restoration sites, with regard to clutch size, hatching, or fledging success. In 2022, Project Area vireos fledged 2.2±1.7 young per pair, which was slightly higher than productivity in 2021 (1.9±1.7 young per pair) but lower than the 16-year average (2.5±0.2, table 9; fig. 10). There was no difference Table 8. Reproductive parameters for nesting Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022. [Nests were categorized based on their locations within Channel, Off-channel, and Restoration sites rather than based on the pair’s territory site category. Standard deviations presented with means. Channel includes Reaches 1, 2, 3a, 3b, and 4, excluding restoration sites. Off-channel includes Upper Pond and Whelan Mitigation, excluding restoration sites. Restoration includes Benet Restoration, College Restoration, and Whelan Restoration. Abbreviations: ±, plus or minus; NA, not applicable; %, percent]

Parameter

Channel Off-channel Restoration

Overall

Nests with eggs

103

128

Eggs laid

295

367

Average clutch size1

3.3±0.6

3.1±0.3

3.2±0.5

Hatchlings

172

214

Nests with hatchlings

Hatching success Eggs2

58%

60%

44%

58%

Nests3

61%

59%

33%

60%

Fledglings

134

164

Nests with fledglings

Hatchlings4

78%

68%

100%

77%

Nests5

78%

69%

100%

77%

Fledging success

1Based on 55 Channel and 12 Off-channel nests with a full clutch (One-way ANOVA: F-ratio=0.97, P=0.33). 2Percent of all eggs that hatched, among all three sites: P=0.68, Fisher’s Exact test. 3Percent of all nests with eggs in which at least one egg hatched, among all three sites: P=0.71, Fisher’s Exact test. 4Percent of all hatchlings that fledged, among all three sites: P=0.25, Fisher’s Exact test. 5Percent of all nests with hatchlings in which at least one young fledged,

among all three sites: P=0.61, Fisher’s Exact test.

in productivity (the number of young fledged per pair) or in the proportion of pairs fledgling one or more young among site types. Overall, 72 percent (55/76) of pairs fledged at least one young in 2022, which was slightly higher than in 2021 (66 percent) but similar to the 16-year average of 74 percent. In 2022, 5 percent of pairs (4/76) successfully fledged two broods, which was lower than the 8 percent observed in 2021 and below the 16-year average of 13 percent. Without the rescue effect of removing Brown-headed Cowbird eggs from parasitized nests, thus allowing them to potentially fledge young, productivity and nest success would have been lower. Had cowbird eggs not been removed from parasitized nests, the fledging rate would have been only 1.8±1.7 vireo young per pair, with only 57 percent of pairs successfully fledging at least one young.

Effects of Treatment on Vireo Reproduction The Treatment Index for fully monitored 2022 vireo territories within the Channel that did not overlap with active restoration averaged 0.07±0.11 (n=59, range, 0–0.49; fig. 11). In 2022, there were three fully monitored vireo territories (5 percent, 3/59) within the Channel that had no treatment. There was no effect of Treatment Index on any measures of nesting effort or productivity (number of nests, number of completed nests, or number of fledglings per pair; table 10; fig. 12).

Table 9. Productivity of nesting Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022. [±, plus or minus; %, percent]

OffRestoration channel

Parameter

Channel

Mixed

Overall

Number of pairs

Average number of young fledged per pair1

2.2±1.8

2.2±1.3

4.0±0.0

1.3±1.5

2.2±1.7

Pairs fledging at least one young2

43 (73%) 9 (75%)

1 (100%)

2 (50%)

55 (72%)

Pairs fledging two broods3

4 (7%)

0 (0%)

4 (5%)

0 (0%)

1One-way ANOVA: F-ratio=0.77, P=0.51. 2P=0.78, Fisher’s Exact test. 3P=1.00, Fisher’s Exact test.

Results 23

Average number of fledglings per pair

6.00 5.00 4.00 3.00 2.00 1.00 0.00

2006

2008

2010

2012

2014

2016

2018

2020

2022

Year

Figure 10. Average number of fledglings per pair (by year) at the San Luis Rey Flood Risk Management Project Area, California, 2006–22. Bars are standard deviations.

Number of territories

0 0.0

0.1

0.2

0.3

0.4

Treatment Index

Figure 11. Treatment Index of monitored vireo territories within the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022. Territories overlapping with active restoration are not included.

0.5

24 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Table 10. Generalized linear model (GLM) results, parameter estimate, standard error (SE), Z statistic, and P-values for Least Bell’s Vireo reproductive parameters as a function of Treatment Index within the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022. [SE, standard error; Z, Z-statistic; P, P-value; <, less than; estimate, parameter estimate]

Variable

Estimate

Number of nests (Intercept)

0.8

0.1

7.9

<0.01

Treatment index

−1.4

1.0

−1.4

0.16

Number of completed nests (Intercept)

0.7

0.1

6.2

<0.01

Treatment index

−0.9

1.0

−1.0

0.34

Number of fledglings 0.8

0.1

7.8

<0.01

Treatment index

−0.9

0.9

−1.0

0.31

4 3 2 1 0.0

0.1

0.2

0.3

0.4

Treatment Index

0.5

Number of fledglings

Number of completed nests

Number of nests

(Intercept)

3 2 1 0

7 6 5 4 3 2 1 0

0.0

0.1

0.2

0.3

0.4

Treatment Index

0.5

0.0

0.1

0.2

0.3

0.4

Treatment Index

0.5

Figure 12. Effect of Treatment Index on Least Bell’s Vireo reproductive parameters (per pair) in A, number of nests per pair; B, number of completed nests per pair; and C, number of fledglings per pair at the San Luis Rey Flood Risk Management Project Area, California, 2022. Lines represent linear regression.

Results 25 We analyzed 2022 nest survival to determine if there was an effect of Treatment Index on DSR (within the Channel only). The best model for DSR was the constant model (table 11). There was some support for the model that included Treatment Index (∆AICc equal to 2); however, an analysis of the odds ratio indicated that it was not a significant contributor to the model (β=−0.01, SE=0.04, odds ratio=0.99, 95-percent confidence interval=0.93–1.07).

Nest Survival and Habitat Use

survival rate (table 12). The best model was the constant DSR model; however, there was support for several of the other models (ΔAICc less than or equal to 2). We looked at the beta estimates for the first two models: the model that included percent cover 0–1 m+1–2 m and the model that included percent cover 1–2 m. For both of these models, the beta values for the percent cover estimates were low (0–0.01), with the confidence interval for the odds ratio spanning one, so we can conclude that percent cover 0–1 m and 1–2 m were not significant contributors to the models (table 13).

We investigated the effect of habitat structure at vireo nest sites on nest survival by constructing a series of models relating the vegetation cover variables to the daily nest Table 11. Logistic regression models for the effects of Treatment Index (within the Channel only) on daily nest survival of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022. [Models are ranked from best to worst based on Akaike’s Information Criterion for small samples (AICc), ΔAICc, and Akaike weights. AICc is based on −2 x loge likelihood, the number of parameters in the model, and sample size. Abbreviations: ΔAICc; difference in AlCc compared with the top model (lowest AlCc)]

Deviance

Number of parameters

AICc

ΔAICc

AICC weight

Constant

330.0

332.0

0.0

0.73

Treatment index

329.9

334.0

2.0

0.27

Model

Table 12. Logistic regression models for the effects of habitat structure on daily nest survival of Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22. [Models are ranked from best to worst based on Akaike’s Information Criterion for small samples (AICc), ΔAICc, and Akaike weights. AICc is based on −2 x loge likelihood, the number of parameters in the model, and sample size. Abbreviations: ΔAICc, difference in AlCc compared with the top model (lowest AlCc); DSR, daily survival rate; +, plus; m, meter]

Deviance

Number of parameters

AICc

ΔAICc

AICc weight

Constant DSR

2,252.7

2,254.7

0.00

0.17

Percent cover 0–1 m+1–2 m

2,248.7

2,254.7

0.04

0.17

Percent cover 1–2 m

2,251.7

2,255.7

1.04

0.10

Percent cover 5–6 m

2,252.3

2,256.3

1.56

0.08

Percent cover 6–7 m

2,252.3

2,256.3

1.60

0.08

Percent cover 3–4 m

2,252.4

2,256.4

1.73

0.07

Percent cover 4–5 m

2,252.5

2,256.5

1.84

0.07

Percent cover 0–1 m

2,252.6

2,256.6

1.88

0.07

Canopy height

2,252.6

2,256.6

1.92

0.07

Percent cover 7–8 m

2,252.6

2,256.6

1.94

0.06

Percent cover 2–3 m

2,252.7

2,256.7

2.00

0.06

Percent cover 2–3 m+3–4 m+ 4–5 m+5–6 m+6–7 m+7–8 m

2,250.9

2,264.9 10.23

0.00

Model

26 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Nest Characteristics

Table 13. Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for models explaining the effect of habitat structure on the daily survival rate of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2006–22.

Channel nests were placed higher in the host plant than Off-channel nests, but there were no other differences in nest placement or host plant size among Channel, Off-channel, and Restoration nests (table 14). Host height, distance to edge of host, and distance from edge of clump were similar in all site types. Within sites, we detected some differences between successful and unsuccessful nests. In the Channel, successful nests were placed closer to the edge of the host plant than unsuccessful nests. The same pattern was discovered in Off-channel nests, with successful nests placed closer to the edge of the host. We also determined that Off-channel successful nests were placed lower in the vegetation, in shorter host plants, and closer to the edge of the vegetation clump than unsuccessful nests.

[Models are in order of best-supported to least-supported. Abbreviations: +, plus; m, meter]

Odds ratio

95-percent CI

Effect

Intercept

3.82

0.15

45.66

33.97–61.39

0–1 m

−0.01

0.01

0.99

0.98–1.00

1–2 m

0.01

1.01

1.00–1.03

Percent cover 0–1 m+1–2 m

Percent cover 1–2 m Intercept

3.73

0.14

41.71

31.59–55.06

1–2 m

0.00

1.00

1.00–1.01

Table 14. Least Bell’s Vireo nest characteristics and results of Kruskal-Wallis and Mann-Whitney U-tests comparing Channel, Off-channel, and Restoration nests and successful versus failed nests at the San Luis Rey Flood Risk Management Project Area, California, 2022. [Standard deviation presented in parentheses. Asterisks indicate a significant difference. N, Sample size; H, Kruskal-Wallis H statistic; P, P-value; U, Mann-Whitney U statistic. Abbreviations: m, meter; <, less than; NA, not applicable; —, no data]

Nest characteristic

Channel

Off-channel

Restoration

Overall Average nest height (m)

1.0 (0.4)

117

0.8 (0.4)

1.3 (0.2)

8.6

0.01*

Average host height (m)

4.1 (2.4)

119

4.1 (3.0)

2.5 (1.5)

4.5

0.11

Average distance to edge of host (m)

0.9 (0.8)

115

0.8 (0.6)

0.4 (0.2)

1.2

0.56

Average distance to edge of clump (m)1

1.3 (0.9)

117

1.1 (0.6)

0.6 (0.4)

3.3

0.2

Channel

Successful

Unsuccessful

Average nest height (m)

1.0 (0.3)

1.1 (0.5)

1,748

0.43

Average host height (m)

3.6 (2.3)

4.3 (2.4)

1,843

0.4

Average distance to edge of host (m)

0.8 (0.9)

1.0 (0.8)

1,866.5

0.08*

Average distance to edge of clump (m)

1.5 (1.0)

1.3 (0.9)

1,460

0.37

Off-channel Average nest height (m)

0.6 (0.2)

1.0 (0.4)

122

<0.01*

Average host height (m)

2.2 (1.5)

5.1 (3.2)

108.5

0.04*

Average distance to edge of host (m)

0.6 (0.6)

0.9 (0.6)

102

0.09*

Average distance to edge of clump (m)

0.8 (0.5)

1.2 (0.6)

108

0.04*

Restoration Average nest height (m)

1.3 (0.2)

—

Average host height (m)

2.5 (1.5)

—

Average distance to edge of host (m)

0.4 (0.2)

—

Average distance to edge of clump (m)

0.6 (0.4)

—

1Clump boundaries were defined where leaves and branches of neighboring plants no longer overlapped.

Results 27 Red/arroyo willow were the species most commonly selected for nesting by vireos in all three site types (50 percent of nests in the Channel, 48 percent in Off-channel sites, and 67 percent in Restoration sites; table 15). Other species commonly used by vireos (greater than 10 percent of all nests) included black willow and mule fat. Vireos used a wider variety of species for nesting in Channel and Off-channel sites (eight and six species, respectively) compared to Restoration sites (two species), although there was limited nesting in Restoration in 2022. Four nests were placed in exotic species in 2022.

Banded Birds We determined banding status for 92 percent (231/251) of adult vireos (134 males, 99 percent of all males and 97 females, 84 percent of all females) observed at the San Luis Rey Flood Risk Management Project Area in 2022. Of these adult vireos, 43 had been banded before the 2022 breeding season, and their identities were verified through resighting or recapture. There were 11 vireos (2 males and 9 females) that could not be identified: 2 because the resight was incomplete and the band combinations could not be confirmed and 9 because they were banded with only a single numbered metal federal band as a nestling (natal), and we were unable to recapture them in 2022 (table 16; appendix 4).

Of the 43 returning adults with a known identity, 42 were banded with full color band combinations before 2022, and 1 was a natal vireo recaptured and color banded in 2022. All banded adults (43) were originally banded in the Project Area (table 16). Adult birds of known age ranged from 1 to 7 years old (table 17).

Emigrants Four vireos that were originally banded in the San Luis Rey River Project Area as nestlings or adults were detected in areas outside of the Project Area in 2022 (tables 16, 18; appendix 5). All vireos were recaptured in 2022 and banded with unique color band combinations at two drainages within MCBCP, at the middle San Luis Rey River (upstream from the Project Area), and at Fallbrook Creek (FNWS; U.S. Geological Survey, unpub. data, 2022).

New Captures A total of 146 vireos were banded for the first time during 2022. These newly banded birds included 8 adults that were captured and banded with a unique color combination and 138 nestlings that were banded with a single numbered dark blue metal federal band. These newly banded vireos were not included in survivorship, fidelity, or movement analyses.

— —

9 5 0 1 1 0 — — —

Black willow

Poison oak (Toxicodendron diversilobum)

Giant reed

Coyote bush (Baccharis pilularis)

Castor bean (Ricinus communis)

Sandbar willow

Fremont cottonwood

Evening primrose (Oenothera elata)

Common fig (Ficus carica)

—

Mule fat

Unsuccessful

Successful

Red/arroyo willow

Host species

Number of Channel nests

[Numbers in parentheses are proportions of total nests. Abbreviation: —, no data]

Total

—

1 (0.01)

2 (0.02)

13 (0.11)

21 (0.18)

60 (0.50)

—

Successful

—

Unsuccessful

Number of Off-channel nests

1 (0.04)

3 (0.12)

—

7 (0.28)

12 (0.48)

Total

—

Successful

Table 15. Host plant species used by Least Bell’s Vireos for nesting at the San Luis Rey Flood Risk Management Project Area, California, 2022.

—

Unsuccessful

Number of Restoration nests Total

—

1 (0.33)

—

2 (0.67)

28 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Results 29 Table 16. Banding status of Least Bell’s Vireos detected at the San Luis Rey Flood Risk Management Project Area, California, 2022, and those that emigrated from the Project Area, 2022. [Project Area includes two natal immigrant vireos. Natal: natal vireos were originally banded as nestlings with a single numbered federal band. Abbreviation: —, no data]

Project area

Banding status

Male

Female

Emigrants

Subtotal

Male

Female

Subtotal

—

Total

Known-identity vireos Banded prior to 2022

Natal, recaptured in 2022

—

Subtotal

Unidentified vireos Natal, not recaptured

—

Incomplete resight

—

Grand total

Table 17. Number of banded adult Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022, by original year banded, age, and sex. [Year originally banded: Year banded with a single numbered metal federal band as a nestling or with a unique color band combination as an adult; Age in 2022: Exact ages determined from single numbered metal federal band observed during recapture; estimated age applies to a bird captured as an unbanded adult; Unknown: Natal vireos banded with single numbered metal federal band or identity unknown because of incomplete resight, so year originally banded was unknown. Abbreviations: yr, year; ≥, greater than or equal to; —, not applicable]

Year originally banded

Age in 2022

2015

San Luis Rey River

Marine Corps Base Camp Pendleton

Total

Male

Female

7 yrs

2016

6 yrs

2017

5 yrs

2018

≥5 yrs

4 yrs

≥4 yrs

3 yrs

2020

≥3 yrs

2 yrs

2021

≥2 yrs

1 yr

Subtotal

—

Unknown

≥1 yr

Total

—

2019

1Two vireos seen with an incomplete resight of the bands so identity and origin are unknown. 2Vireos seen with a single metal dark blue numbered band, indicating that they were originally banded on the San Luis Rey River before 2022. 3Two vireos seen with a single metal gold numbered band, indicating that they were originally banded at Marine Corps Base Camp Pendleton before 2022.

Table 18. Number of emigrant Least Bell’s Vireos detected outside of the Project Area that originated in the San Luis Rey Flood Risk Management Project Area, California, by original year banded, age, 2022 location, and sex. [Year originally banded: Year banded with a single numbered metal federal band as a nestling or with a unique color band combination as an adult; MCBCP: Marine Corps Base Camp Pendleton; Middle San Luis Rey River: Upstream from the Project Area, between College Boulevard and Interstate 15. Abbreviations: yr, year, —, no data]

Year originally banded

Age in 2022

2018 2019

2022 location drainage MCBCP drainages (female)

Middle San Luis Rey River (male)

Other drainages Total (male)

4 yrs

—

3 yrs

—

1 2 4

2021

1 yr

—

Total

—

1Detected at Aliso Creek. 2Detected at Santa Margarita River. 3Detected at Fallbrook Creek, Fallbrook Naval Weapons Station.

30 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Survivorship, Fidelity, and Movement Survivorship Models—Adults-Only The best model explaining adult annual survivorship included effects of sex and year on adult survival (table 19) and the effect of sex on detection probability. Detection probability was somewhat higher for males (0.88) than for females (0.76) but was high for both sexes. Male survivorship was higher than female survivorship, and survivorship varied by year, with significantly lower survival in 2010–11, 2011–12, 2019–20, and 2020–21, relative to the reference (2006–07, tables 20, 21). We found little support for models that included bio-year precipitation, suggesting that precipitation on the breeding grounds was not the primary driver of annual fluctuations in survivorship in this system (table 19). We used the top model to calculate annual survivorship for adult males and adult females (table 21). Annual survival for females was consistently lower than for males, averaging 59±12 percent (range of 37–71 percent). For males, annual survivorship averaged 66±11 percent (range of 44–79 percent).

Survivorship Models—Adults and First-Year Vireos The best supported model for vireo survival was the model that included the effects of age and year on survival (table 22). Analysis of the odds ratio showed that survival of adult vireos was significantly higher than that of first-year vireos and that survival varied by year for both age classes, with higher survival in 2008–09 and lower survival in 2010–12 and 2019–22, relative to the reference (2006–07; table 23). Survival estimates for 2021–22 were low relative to previous years (adult survival was estimated at 50 percent and first-year survival was 7 percent). Average annual

survival across all years for adults was 67±12 percent (range of 50–82 percent) and 15±6 percent (range of 7–25 percent; table 24) for first year birds.

First-year Dispersal 2022 Three first-year vireos (all male) that fledged from nests in the Project Area in 2021 were recaptured in 2022 (table 25). Of these vireos, one was recaptured within the Project Area and two were recaptured outside of the Project Area (one at Fallbrook Creek and one at the middle San Luis Rey River, upstream from the Project Area between College Boulevard and Interstate 15 [U.S. Geological Survey, unpub. data, 2022]; table 25; appendix 6). Average dispersal distance by first-year vireos was 6.7±7.4 km (range 2.3–15.3 km). The longest dispersal distance was 15.3 km by a male vireo recaptured at Fallbrook Creek, FNWS.

First-year Dispersal 2007–22 We examined the return locations of first-year vireos in 2022 and compared them to dispersal observed in previous years. From 2007 to 2011, the majority (67–86 percent) of returning first-year vireos returned to the Project Area (table 25). As habitat in the Project Area became more limited following the commencement of mowing in 2012, proportionately more birds dispersed to sites outside of the Project Area, a trend that continued until about 2017. Since 2018, the proportion of first-year vireos returning to the Project Area has been generally increasing, although still below the proportions observed before vegetation mowing. In 2022, the total number of returning first-year vireos was low, and only one of the three first-year vireos returned to the Project Area.

Table 19. Survivorship models for the effects of sex, year, and precipitation on adult survival for Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22. [The effect of sex on detection probability was included in all models. Models are ranked from best to worst based on Akaike’s Information Criterion for small samples (AICc), ΔAICc, and Akaike weights. AICc is based on −2 x loge likelihood, the number of parameters in the model, and sample size. Abbreviation: +, plus]

Deviance

Number of parameters

AICc

ΔAICc

AICC weight

Sex+year

260,287.8

263,454.8

0.0

0.73

Year

260,292.1

263,457.2

2.3

0.23

Sex+precipitation 260,322.2

263,461.0

6.2

0.03

Precipitation

260,327.5

263,464.2

9.4

0.01

Sex

260,369.9

263,506.7

51.9

0.00

Constant

260,375.0

263,509.8

55.0

0.00

Model

Results 31 Table 20. Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for the top model explaining annual survivorship (Phi) of adult Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22.

Table 21. Annual survivorship for adult female and adult male Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22. [Survivorship and estimates were derived from the top model]

[The Intercept (reference) represents female vireos, 2006–07. All other effects values are the difference between that parameter and the Intercept. Abbreviation: +, plus]

Effect

Odds ratio

95-percent CI

Sex+year Intercept

0.89

0.35

2.44

1.23–4.86

Male1

0.31

0.15

1.36

1.02–1.80

2007–08

−0.09

0.47

0.91

0.37–2.27

2008–09

0.11

0.41

1.12

0.50–2.48

2009–10

−0.26

0.38

0.77

0.37–1.62

2010–111

−0.94

0.37

0.39

0.19–0.80

2011–121

−1.35

0.37

0.26

0.12–0.54

2012–13

−0.13

0.42

0.88

0.38–2.00

2013–14

−0.15

0.41

0.86

0.39–1.91

2014–15

−0.22

0.40

0.80

0.36–1.77

2015–16

−0.70

0.39

0.49

0.23–1.07

2016–17

−0.23

0.42

0.79

0.35–1.80

2017–18

−0.75

0.39

0.47

0.22–1.01

2018–19

−0.25

0.39

0.78

0.37–1.67

2019–201

−1.04

0.37

0.35

0.17–0.74

2020–211

−1.42

0.37

0.24

0.12–0.50

2021–22

−0.78

0.43

0.46

0.20–1.06

Years

Adult female survivorship

Adult male survivorship

2006–07

0.71

0.77

2007–08

0.69

0.75

2008–09

0.73

0.79

2009–10

0.65

0.72

2010–11

0.49

0.56

2011–12

0.39

0.46

2012–13

0.68

0.74

2013–14

0.68

0.74

2014–15

0.66

0.73

2015–16

0.55

0.62

2016–17

0.66

0.72

2017–18

0.53

0.61

2018–19

0.66

0.72

2019–20

0.46

0.54

2020–21

0.37

0.44

2021–22

0.53

0.60

1Significant effect.

Table 22. Survivorship models for the effect of age, year, and precipitation on survival of Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22. [Detection probability was modeled as constant. Models are ranked from best to worst based on Akaike’s Information Criterion for small samples (AICc), ΔAICc, and Akaike weights. AICc is based on −2 x loge likelihood, the number of parameters in the model, and sample size. Abbreviations: ΔAICc; difference in AlCc compared with the top model (lowest AlCc)]

Deviance

Number of parameters

AICc

ΔAICc

Age + year

3,513.6

6,678.5

0.0

1.00

Age + precipitation

3,626.1

6,762.8

84.3

0.00

Age

3,646.7

6,781.4

102.9

0.00

Year

4,714.9

7,877.8 1,199.3

0.00

Model

AICC weight

32 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Table 23. Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for the top model explaining annual survivorship of Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22. [The Intercept (reference) represents first-year vireos, 2006–07. All other effects values are the difference between that parameter and the Intercept]

Effect

Odds ratio

95-percent CI

Age+year Intercept

−1.52

0.17

0.22

0.16–0.31

Adults1

2.57

0.08

13.06

11.19–15.25

2007–08

−0.25

0.24

0.78

0.49–1.25

2008–091

0.45

0.21

1.56

1.04–2.35

2009–10

−0.07

0.22

0.94

0.61–1.45

2010–111

−0.84

0.22

0.43

0.28–0.67

2011–121

−0.97

0.23

0.38

0.24–0.60

2012–13

0.17

0.25

1.18

0.72–1.93

2013–14

−0.35

0.23

0.70

0.45–1.10

2014–15

0.06

0.27

1.06

0.62–1.80

2015–16

−0.27

0.24

0.76

0.48–1.21

2016–17

−0.11

0.24

0.90

0.56–1.44

2017–18

0.09

0.22

1.10

0.71–1.69

2018–19

−0.19

0.23

0.82

0.52–1.29

2019–201

−0.55

0.22

0.58

0.37–0.90

2020–211

−0.92

0.25

0.40

0.25–0.64

2021–221

−1.05

0.28

0.35

0.20–0.60

1Significant effect.

Table 24. Annual survivorship for adult and first-year Least Bell’s Vireos at the San Luis Rey Flood Risk Project Area, California, 2006–22. [%, percent]

Years

Adult survivorship

First-year survivorship

2006–07

74%

18%

2007–08

69%

15%

2008–09

82%

25%

2009–10

73%

17%

2010–11

55%

2011–12

52%

2012–13

77%

20%

2013–14

67%

13%

2014–15

75%

19%

2015–16

68%

14%

2016–17

72%

16%

2017–18

76%

19%

2018–19

70%

15%

2019–20

62%

11%

2020–21

53%

2021–22

50%

Results 33 Table 25. Number of first-year Least Bell's Vireos that originated at the San Luis Rey Flood Risk Management Project Area and returned to the Project Area or returned to areas outside of the Project Area, 2007–22, by year and sex. [Numbers in parentheses indicate the percentage of returning first-year vireos that returned to that area. Middle San Luis Rey River is upstream from the Project Area, between College Boulevard and Interstate 15. Abbreviations: %, percent, —, no data]

Drainage location

Middle San Luis Rey River

Other drainages

Project Area total

Male Female Male Female Male Female

18 (75%)

— 7

—

Male Female 2007

Marine Corps Base Camp Pendleton

San Luis Rey Project Area

Return year

2008

20 (67%)

2009

59 (79%)

Outside Project Area total

—

6 (25%)

—

10 (33%)

16 (21%)

2010

18 (69%)

—

8 (31%)

2011

18 (86%)

—

3 (14%)

2012

11 (61%)

—

7 (39%)

2013

13 (57%)

—

10 (43%)

2014

5 (31%)

—

11 (69%)

2015

5 (50%)

—

5 (50%)

14 (41%)

—

20 (59%) 10 (43%) 23 (32%)

2016

2017

13 (57%)

—

101

2018

49 (68%)

111

2019

10 (53%)

—

121

—

9 (47%)

2020

24 (60%)

—

16 (40%)

2021

10 (63%)

—

121

6 (37%)

2022

—

1 (33%)

—

131

—

2 (67%)

Total

192

288 (64%)

162 (36%)

1Detected at Elanus Creek, San Diego County. 2One detected at the Santa Ana River in Orange County, one detected at San Dieguito River, and one detected at Agua Hedionda, San Diego County. 3Detected at the San Diego River, San Diego County. 4Detected at Chollas Creek, San Diego County. 5Detected at Bonita Creek, Orange County. 6Detected at the San Dieguito River, San Diego County. 7One detected at the San Gabriel River, Los Angeles County, one detected at the Sweetwater River, and one detected at Moody Creek, San Diego County. 8Detected at Escondido Creek, San Diego County. 9One detected at the Tijuana River and one detected at Otay River, San Diego County. 10Detected at the Tijuana River, San Diego County. 11Detected at the upper Santa Margarita River, upstream (12 kilometers) from Marine Corps Base Camp Pendleton. 12Detected at Whelan Lake, San Diego County. 13Detected at Fallbrook Creek, Fallbrook Naval Weapons Station.

34 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Southwestern Willow Flycatcher

Adult Site Fidelity Resighting banded birds allowed us to identify individuals that either returned to the same territory they used in a previous year (within 100 m) or moved to a different location. There were 41 adult vireos (37 males and 4 females) identified in 2021 at the Project Area that were resighted in 2022 (appendix 7). Of the 41 returning adults, 27 vireos (66 percent of all adults; 73 percent of males; no females) occupied a territory in 2022 that they had defended in 2021 (within 100 m; appendix 7). Six vireos (15 percent of all adults; 4 males, 11 percent of males; 2 females, 50 percent of females) detected in 2022 returned to areas adjacent to their previous territories (within 300 m). The remaining 8 vireos (20 percent of all adults; 6 males, 16 percent of males; 2 females, 50 percent of females) detected in 2022 returned to areas more than 300 m from their previous territories. The average distance moved by returning 2022 adult vireos was 0.3±0.7 km, range 0.0–4.0 km for males; 0.6±0.4 km, range 0.2–1.1 km for females.

Effect of Treatment on Adult Site Fidelity Because we were interested in the cumulative effect of vegetation treatment on the movement of returning banded vireos over the years, we performed a linear regression on the distance moved by banded vireos as a function of the expected Treatment Index for the vireos’ 2021 territories (that is, the Treatment Index in 2022 if the bird had occupied exactly the same territory boundaries as in 2021). The residuals were skewed, so we log-transformed the data for analysis. We restricted our analysis to birds nesting in the Channel to avoid the confounding effects of passive habitat restoration that occurred Off-channel in 2017. We detected no effect of expected Treatment Index on distance moved between 2021 and 2022 (P=0.86, table 26). Table 26. Generalized linear model (GLM) results, parameter estimate, standard error (SE), t statistic, and P-values (P) for banded Least Bell’s Vireo movement (log-transformed) as a function of the expected Treatment Index for 2021 territories at the San Luis Rey Flood Risk Management Project Area, California, 2022. [<, less than]

Variable

Estimate

Intercept

4.1

0.5

8.6

<0.01

Expected Treatment Index

0.4

2.1

0.2

0.86

Population Size and Distribution Four transient Willow Flycatchers of unknown subspecies were detected within the Project Area between May 24 and June 9, 2022. Two transients were detected in Reach 1, one in Reach 3a, and one in Pilgrim Pond (appendix 8, figs. 8.1‒8.2). No flycatcher pairs were detected within the Project Area in 2022.

Habitat Characteristics Two transient flycatchers used mixed willow riparian habitat, one used willow-cottonwood habitat, and one used riparian scrub habitat in 2022 (table 27). The dominant native species in three of the flycatcher locations were red/arroyo willow. One flycatcher was detected in habitat dominated by mule fat. Two flycatchers were located in habitat with 5–50-percent exotic vegetation and two were in habitat with less than 5-percent exotic vegetation. Poison hemlock was the most common exotic species at flycatcher locations. Flycatchers were detected between 0 and 1.3 km away from surface water.

Vegetation Study A total of 46 transects (528 sampling points) were sampled in the Project Area in 2022 (table 28; appendix 9, figs. 9.1–9.4). Seventy-one percent (378/528) of points were in the Channel and 22 percent (115/528) were located Off-channel in Upper Pond. The remaining 7 percent (35/528) of points were in the Whelan Restoration site. The number of points per transect varied between 4 and 18. One point was in a transient camp and was not sampled in 2022. GPS coordinates for the start and end point of each transect are provided in appendix 10.

Vegetation Structure and Composition There was more foliage cover below 2 m in the Channel than in Upper Pond and Whelan Restoration (fig. 13). Above 2 m, foliage cover was similar at the Channel and Whelan Restoration and was greater than at Upper Pond. Overall, foliage cover was low above 5 m but was highest in the Channel. Average live canopy height and maximum canopy height (including live and dead vegetation) was higher in the Channel (5.6±3.4 m) compared to Upper Pond (4.7±2.9 m), and Whelan Restoration (4.6±1.9 m).

Results 35 Table 27. Habitat characteristics of transient Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area, California, 2022. [ID, identification; km, kilometer, <, less than, —, no data]

Bird ID

Date first detected

BT01F

May 24

Mixed willow

Habitat type

Dominant species

Percent cover exotics

Dominant exotics

Distance to surface water (km)

Red/arroyo willow

5–50

Poison hemlock

1.3

BT03F

May 24

Willow-cottonwood Red/arroyo willow

—

0.4

DO01F

May 24

Riparian scrub

Mule fat

5–50

Poison hemlock

1.0

WH01F

July 9

Mixed willow

Red/arroyo willow

—

0.3

Table 28. Number of vegetation transects and sampling points at the San Luis Rey Flood Risk Management Project Area, California, 2022. Site type

Transects

Sampling points

Channel

378

Upper Pond

115

Whelan Restoration

Total

528

EXPLANATION Channel

7–8

Upper Pond Whelan Restoration

Height class, in meters

6–7

5–6

4–5

3–4

2–3

1–2

0–1 0

Percent foliage cover

Figure 13. Average percent foliage cover by height class at the San Luis Rey Flood Risk Management Project Area, California, 2022. Bars are standard deviations.

36 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Vegetation composition in the Channel differed from that in Upper Pond and Whelan Restoration (fig. 14). Tree cover, dominated by red/arroyo willow, was higher in the Channel (43 percent) and Whelan Restoration (44 percent) compared to Upper Pond (22 percent). In contrast, shrub cover was substantially lower in the Channel (12 percent) than in Upper Pond (43 percent) and Whelan Restoration (32 percent). Mule fat was the dominant shrub species at all three sites. Herbaceous cover was at least 2 times greater in both the Channel (22 percent) and Upper Pond (19 percent) than in Whelan Restoration (8 percent). Marsh species such as bulrush (Scirpus sp.), sedge (Carex sp.), and cattail (Typha latifolia) comprised 59 percent of the total herbaceous cover in the Channel. Exotic species cover also was higher in the Channel (19 percent) than at Whelan Restoration (11 percent) and Upper Pond (4 percent). Giant reed comprised 42 percent of the total exotic cover in the Channel. Dead woody cover was highest in Upper Pond (12 percent) compared to Whelan Restoration (5 percent) and the Channel (4 percent). Dead vegetation in Whelan Restoration was primarily red/arroyo willow, whereas most dead vegetation at Upper Pond and the Channel could not be identified.

Vegetation Changes 2006–22 When we examined the cumulative effects of management activities on vegetation structure over time at the Project Area, we detected significant changes from 2006 to 2022 in the Channel (fig. 15). We detected significant negative trends for all height classes above 2 m (2–3 m: R2=0.21,

Percent foliage

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

P=0.03; 3–4 m: R2=0.14, P=0.07; 4–5 m: R2=0.12, P=0.09; 5–6 m: R2=0.18, P=0.05; 6–7 m: R2=0.30, P=0.01; 7–8 m: R2=0.37, P=0.00; greater than 8 m: R2=0.60, P=0.00 [fig. 15]). Decreases were steeper during 2008–13 and 2014–16 than during 2006–08. Beginning in 2016, percent foliage cover increased at all height classes. Herbaceous cover doubled between 2016 (13 percent) and 2021 (29 percent) and has remained high through 2022 (22 percent). Although increases were observed at all height classes after 2016, overall percent foliage cover has remained lower than levels detected before 2009. At Upper Pond, especially in the lower height classes, we detected very few trends or patterns with regard to vegetation cover from 2006 to 2022. Vegetation under 4 m has fluctuated annually with no discernable pattern and, in 2022, had cover levels comparable to what was observed in 2006. For the higher strata, which represented only a small fraction of the overall cover at Upper Pond, we detected a significant negative trend from 2006 to 2022 at the 4–5 m height class (4–5 m: R2=0.31, P=0.01 [fig. 16]). At Whelan Restoration, we detected a significant positive trend for the 5–6 m height class (R2=0.12, P=0.09) and a significant negative trend for the highest height class (above 8 m [R2=0.34, P=0.01]); however, overall cover in these strata make up only a small percentage of the total cover at this site. The general pattern observed at Whelan Restoration has been one of decline and recovery, with foliage cover declining at all height classes from 2006 to 2010, after which cover has steadily increased at most height classes. Current (2022) cover is similar to 2006 levels for most height classes (fig. 17).

Channel

Upper Pond

Whelan Restoration

EXPLANATION Tree Shrub

Herb Exotic

Dead

Figure 14. Percent of total foliage cover by vegetation type at Channel, Upper Pond, and Whelan Restoration sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Results 37

EXPLANATION Channel 2006 Channel 2007

7–8

Channel 2008 Channel 2009

6–7

Channel 2010

Height class, in meters

Channel 2011 Channel 2012

5–6

Channel 2013 Channel 2014

4–5

Channel 2015 Channel 2016 Channel 2017

3–4

Channel 2018 Channel 2019

2–3

Channel 2020 Channel 2021 Channel 2022

1–2

0–1 0

100

Percent foliage cover

Figure 15. Average percent foliage cover by height class (meters) for the Channel at the San Luis Rey Flood Risk Management Project Area, 2006–22. Bars are standard deviations. Asterisks (*) indicate statistically significant trends (P≤0.10, linear regression).

38 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

EXPLANATION Upper Pond 2006 Upper Pond 2007

7–8

Upper Pond 2008 Upper Pond 2009 6–7

Upper Pond 2010

Height class, in meters

Upper Pond 2011 Upper Pond 2012

5–6

Upper Pond 2013 Upper Pond 2014

4–5

Upper Pond 2015 Upper Pond 2016 Upper Pond 2017

3–4

Upper Pond 2018 Upper Pond 2019

2–3

Upper Pond 2020 Upper Pond 2021 Upper Pond 2022

1–2

0–1 0

Percent foliage cover

Figure 16. Average percent foliage cover by height class (meters) for Upper Pond at the San Luis Rey Flood Risk Management Project Area, 2006–22. Bars are standard deviations. Asterisks (*) indicate statistically significant trends 2006–22 (P≤0.10, linear regression).

Results 39

EXPLANATION Whelan Mitigation 2006 Whelan Mitigation 2007

7–8

Whelan Mitigation 2008 Whelan Mitigation 2009

6–7

Whelan Mitigation 2010

Height class, in meters

Whelan Mitigation 2011 Whelan Mitigation 2012

5–6

Whelan Mitigation 2013 Whelan Restoration 2014

4–5

Whelan Restoration 2015 Whelan Restoration 2016 Whelan Restoration 2017

3–4

Whelan Restoration 2018 Whelan Restoration 2019

2–3

Whelan Restoration 2020 Whelan Restoration 2021 Whelan Restoration 2022

1–2

0–1 0

Percent foliage cover

Figure 17. Average percent foliage cover by height class (meters) for Whelan Restoration and Whelan Mitigation at the San Luis Rey Flood Risk Management Project Area, 2006–22. Vegetation points are the same throughout; however, Whelan Restoration was referred to as “Whelan Mitigation” before 2014. Bars are standard deviations. Asterisks (*) indicate statistically significant trends 2006–22 (P≤0.10, linear regression).

40 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Vireo Habitat Use We measured vegetation characteristics at 35 nest plots and 35 territory plots (280 sampling points) within the Channel and 9 nest and 9 territory plots (72 sampling points) at Upper Pond. In the Channel, we determined that vireos were selective with regard to territory, establishing territories in areas with greater foliage cover between 3 and 6 m and less cover below 2 m compared to available habitat. With regard to nest selection, we determined that vireos selected nest sites within their territories largely at random, with the exception

B >8

EXPLANATION

7–8

Territory

6–7

Available

Height class, in meters

that nest sites had more foliage cover between 2 and 3 m than in the rest of the territory (fig. 18). In Upper Pond, the pattern was less clear. Vireos were generally less selective with regard to territory establishment compared to Channel birds, occupying territories with more cover below 1 m and between 5 and 6 m than was available. However, the trend was reversed for nest site selection, with vireos selecting nest sites with less cover below 1 m and between 5 and 6 m relative to available habitat within the territory (fig. 19).

5–6 4–5 3–4 2–3

Nest

6–7

Territory

5–6 4–5 3–4 2–3

1–2

0–1

0–1 0

Percent foliage cover

EXPLANATION

7–8

100

Percent foliage cover

100

Figure 18. A, Average percent foliage cover for territory plots (n=35) compared to vegetation transects (n=31; shown as “available”); and B, nest plots (n=35) compared to territory plots (n=35) by height class in the Channel at the San Luis Rey Flood Risk Management Project Area, California, 2022. Bars are standard deviations. Asterisks (*) denote significant differences between plots by height class (P ≤ 0.10).

B >8 7–8

Territory

6–7

Height class, in meters

EXPLANATION Available

5–6 4–5 3–4 2–3

Nest

6–7

Territory

5–6 4–5 3–4 2–3

1–2

0–1

0–1 0

Percent foliage cover

100

EXPLANATION

7–8

Percent foliage cover

100

Figure 19. A, Average percent foliage cover for territory plots (n=9) compared to vegetation transects (n=13; shown as “available”); and B, nest plots (n=9) compared to territory plots (n=9) by height class in the Upper Pond at the San Luis Rey Flood Risk Management Project Area, California, 2022. Bars are standard deviations. Asterisks (*) denote significant differences between plots by height class (P≤0.10).

Discussion 41

Discussion The Least Bell’s Vireo population (133 territories) at the San Luis Rey Flood Risk Management Project Area increased by 9 percent between 2021 and 2022 and was similar to populations observed in recent years (130 in 2018, 128 in 2019, and 122 in 2021). From the time surveys were initiated in 1984, the population grew steadily until it peaked in 2009, likely the result of an increase in suitable riparian habitat during the 1990s, cowbird control, and high productivity of nesting pairs (Kus and Whitfield, 2005). After the 2009 peak, the population experienced several years of declines before reaching a low of 76 territories in 2012, after which it hovered for 5 years between 78 and 103 territories. In 2018, the population increased and has remained higher (greater than 120 territories) since that time, including a population spike in 2020, likely in response to high productivity in 2019. During the past 5 years, the vireo population in the Project Area has largely tracked regional trends. However, in 2022, we saw a similar increase (4 percent) at MCBCP (U.S. Geological Survey, unpub. data, 2022) but different trends on other drainages with a 10-percent decrease (one territory) at Marine Corps Air Station and an 11-percent decrease (two territories) on the Otay River (K. Ferree, San Diego Natural History Museum, written commun., 2022). Using the Treatment Index to create a quantitative snapshot in time of the vegetation removal from 2006 through the present incorporates spatial and temporal variability in vegetation structure and provides a useful approach for explaining vireos’ reproductive and behavioral responses to habitat alteration. In 2022, like in 2015–21, our analyses did not reveal a relationship between the Treatment Index and reproductive parameters, such as the number of fledglings produced per pair. Vireo territory selection, however, continued to respond to vegetation removal as represented by the Treatment Index. Since 2015, we have determined that vireos have selected territories in less treated habitat and moved to avoid heavily treated areas. In 2022, the average Treatment Index of unoccupied habitat was more than four times that of occupied habitat, indicating an avoidance of more recently treated habitat because vireos selected territories along the Channel edges that have been less affected by mowing. However, unlike in 2016 and 2017, but similar to 2018–21, we did not find an effect of territory treatment on the movement of individual banded birds between 2021 and 2022. Our analysis of vireo habitat supported the finding that vireos tend to avoid more recently and heavily treated sections of the river when selecting their territories. In the Channel, most of the herbaceous and exotic growth occurred in the middle, where vegetation has been mowed. Vireos tended to avoid the middle of the Channel and instead limited their territories to the edges, where more mature, woody vegetation persisted. Vireo territories in the Channel had less total cover below 2 m and more cover at 3–6 m compared to what was available, which describes the strips of mature habitat along the edges of the Channel. Within territories,

however, Channel vireos placed nests largely at random, selecting nest sites that had slightly more cover between 2 and 3 m, but were otherwise similar to the rest of the territory in vegetation structure. Similar to 2021 but in contrast to historical observations, in Upper Pond, vireos were somewhat selective with regard to territory selection, preferring territories with greater cover from 0 to 1 and from 5 to 6 m. Within territories, however, vireos selected nest sites with less cover in those same height classes. Before 2021, vireos selected territories largely at random and within territories selected nest sites with more cover. Upper Pond continues to have the highest proportion of dead cover compared to Whelan and the Channel, and vireos are likely avoiding the most drought-affected areas dominated by dead vegetation and selecting territories in habitat that is recovering. Since mowing of the Channel began in 2005, we have documented a steady decline in total percent foliage cover at all height classes above 2 m. Steeper annual declines in percent cover coincided with large mowing events. Herbaceous (native and exotic) cover recovered quickly after mowing and made up the bulk of the vegetation in the Channel in 2022, in contrast to Upper Pond and Whelan Restoration, which had a greater proportion of shrub cover. In general, shrub cover has not recovered in the Channel (and was limited to the Channel edges) because significant negative trends were detected for all height classes over 2 m. We detected fewer significant negative trends from 2006 to 2022 in percent foliage cover at Upper Pond and Whelan Restoration. At Upper Pond, we documented annual fluctuations, likely primarily in response to environmental conditions, such as variations in annual precipitation. At Whelan Restoration, we documented an initial decline at most height classes, likely in response to drought, followed by steady increases in vegetation cover as it has recovered to 2006 levels. Nest success was similar in 2022 (38 percent of completed nests successful) and 2021 (39 percent of completed nests successful) and did not differ by site type. Vireo breeding productivity was slightly higher in 2022 (2.2 young per pair) compared to 2021 (1.9 young per pair) but remained lower than the 16-year average. Double-brooding was low for the third year in a row, which is well below the long-term average. Lower breeding productivity in 2022 is likely, in part, a response to increased cowbird parasitism and low precipitation (20.0 centimeters [cm] for the 2021–22 bio-year). Precipitation influences primary productivity, with drier years producing limited annual vegetation growth, which limits cover available for foraging, arthropod abundance, and nesting cover for vireos. We have documented strong links between vireo productivity and precipitation since 2006, with years of highest breeding productivity (greater than 2.5 young per pair) occurring in years with higher precipitation (greater than 30 cm, 2008, 2010, 2017, and 2019) and years of lowest breeding productivity (less than 2 young per pair) occurring in years with lower precipitation (less than 15 cm, 2014 and 2021).

42 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Parasitism by Brown-headed Cowbirds has increased dramatically during the past 2 years. With only two exceptions (2009 and 2014), before 2021, we had never documented more than seven instances of cowbird parasitism per year in the Project Area, and between 2015 and 2019, the maximum number of nests parasitized in a year was two. In 2021, however, we documented 35 instances of parasitism. In 2021, we attributed the increase in cowbird parasitism to a delay in the traps being opened. Cowbird trapping typically begins on April 1 of each year, but in 2021, traps in the Project Area were not opened until late April. Historically, the majority of cowbirds have been captured and removed from the population during the month of April (J.Sexton, T.W. Biological Services, written commun., 2021). Having the traps closed during April likely allowed cowbirds to become established on the river during that time. In 2022, although the traps in the Project Area were opened on time, there was a delay in opening traps at MCBCP, where cowbird traps were not opened until late May (B. Leatherman, Leatherman BioConsulting, Inc., written commun., 2022). Although no instances of parasitism were documented in vireo monitoring areas at MCBCP, a male vireo was seen outside of the monitoring sites feeding a cowbird fledgling. In addition, cowbirds were detected on a higher proportion of MCBCP surveys than in any other year (45 percent of surveys in 2022, compared with the 2006–21 average of 14 percent of surveys; U.S. Geological Survey, unpub. data, 2022). It is possible that the delay in opening traps at MCBCP may have contributed to the high rates of parasitism observed in the Project Area. Banding of vireos with unique color combinations (starting in 2006) has allowed us to estimate adult and first-year survival rates and examine adult and first-year movement between and within years at the Project Area and across drainages throughout southern California. Since 2020, however, the number of banded birds in the Project Area has been declining. In 2022, we documented only 52 banded vireos, which is down from 108 in 2019 and 2020. This decrease is, in part, a result of our reduced effort to band unmarked adults, but it does not explain why we also continue to have fewer banded birds in older age classes. In 2022, we did not have any birds older than 7 years. Of the banded birds in the Project Area in 2022, only eight (15 percent) were banded before 2019. There has been a steady decline in the oldest age classes since 2017. Recent annual survival estimates reflect this trend, with lower-than-average estimates of survival for the last 3 years. Adult vireo site fidelity, although remaining high (61 percent), also has been declining since 2019, when 75 percent of banded adults returned to within 100 m of their previous year’s territory.

Beginning in 2012, and especially between 2014 and 2016, we detected a shift in the proportion of first-year vireos returning to the Project Area to breed, with more birds dispersing to areas outside of the Project Area. This shift may be attributed to a decrease in available habitat within the Project Area, likely a result of increased vegetation removal in 2014 and 2015 coupled with drought conditions. Between 2017 and 2021 the trend shifted, with first-year vireos more likely to return to the Project Area for breeding. This shift also coincided with increased precipitation in 3 of the 5 years and more moderate mowing, which has likely contributed to an increase in habitat available for new breeders in the Project Area. In 2022, however, we documented only one returning first-year vireo, whereas two dispersed out of the Project Area for a first-year return rate of just 33 percent. The low return of first-year birds in 2022 was likely influenced by poor capture success of natal vireos. There were nine birds observed with a single blue metal band (identifying that they were banded as nestlings in the Project Area) that we were unable to capture and identify this year. Many of these birds were likely first-year vireos.

Southwestern Willow Flycatcher There were not any breeding pairs of flycatchers detected in 2022; this was the 16th year that the Project Area has not been occupied by Willow Flycatchers since it was colonized and monitoring began in 2000 (Kus and Rourke, 2005). It is unknown why flycatchers did not breed in the Project Area, although conditions in the historical flycatcher territories have been gradually changing since 2006, when three pairs occupied the Project Area. At Whelan Restoration in particular, large canopy trees such as red/arroyo willows and black willows have died. This historic breeding habitat is likely still too dry, sparse, and open for flycatchers. Vegetation clearing does not appear to have played a role in the decline of the flycatcher in the San Luis Rey Flood Risk Management Project Area population. More likely explanations, in addition to habitat change, include other life history factors, such as first-year or adult mortality experienced during migration or on the wintering grounds. At nearby Marine Corps Base Camp Pendleton, the resident population of flycatchers also declined for 10 years, from 26 individuals in 2007 to 0 individuals in 2017 (S. Howell, U.S. Geological Survey, unpub. data, 2017). In 2018, a social attraction study commenced at Camp Pendleton. In 2018–21, there were between one and three resident breeders documented; however, there were no breeders documented in 2022 (S. Howell, U.S. Geological Survey, unpub. data, 2018, 2019, 2020, 2021, 2022). Collection of more information is warranted for this declining species.

References Cited 43

Conclusion The vireo population increased slightly in 2022 and was comparable to population levels during the past several years, which have been high relative to the long-term averages. Nest success and productivity, although higher than in 2021, remain lower than average, which is likely in response to increased levels of cowbird parasitism and lower-than-average precipitation in the preceding winter. Although we continued to find no effect of vegetation treatment on measures of reproductive success, we did find a strong effect of treatment on vireo territory selection. For the past 8 years, we have documented an avoidance of recently treated habitat, even in years in which there was no treatment, indicating that vireos require more than 1 year of habitat recovery between mowing events for territory selection. Mowing of the channel has resulted in two distinct habitat types, neither of which is ideal for vireos. The herbaceous middle of the Channel is too dense in the lower height classes, lacks canopy cover, and has less woody substrate that vireos prefer for nesting. The edges of the Channel are possibly becoming too mature, lacking cover diversity in the lower height classes. A gradient in habitat successional stages within the Project Area and increased cowbird trapping could help to ensure that vireos will be able to continue breeding successfully in the Project Area.

References Cited Burnham, K.P., and Anderson, D.R., 2002, Model selection and multi-model inference—A practical information-theoretic approach (2d ed.): New York, Springer-Verlag. Daubenmire, R.F., 1959, A canopy-coverage method of vegetational analysis: Northwest Science, v. 33, p. 43–64. Dinsmore, S.J., White, G.C., and Knopf, F.L., 2002, Advanced techniques for modeling avian nest survival: Ecology, v. 83, no. 12, p. 3476–3488. [Available at https://doi.org/10.1890/ 0012-9658(2002)083[3476:ATFMAN]2.0.CO;2.] Environmental Systems Research Institute, 2020, ArcGIS Desktop—Release 10.8.1: Redlands, Calif., Environmental Systems Research Institute. Environmental Systems Research Institute, 2022a, Field Maps release 22.3.1: Redlands, Calif., Environmental Systems Research Institute. Environmental Systems Research Institute, 2022b, Survey123 release 3.17.68: Redlands, Calif., Environmental Systems Research Institute.

Ferree, K., and Kus, B.E., 2007, Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2006 Data summary: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Ferree, K., and Kus, B.E., 2008a, Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2007 Annual data summary: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Ferree, K., and Kus, B.E., 2008b, Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2008 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Ferree, K., Allen, L.D., Blundell, M.A., Kus, B.E., and Lynn, S., 2010a, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2009 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Ferree, K., Allen, L.D., Kus, B.E., and Lynn, S., 2010b, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2010 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Ferree, K., Allen, L.D., Kus, B.E., and Lynn, S., 2011, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2011 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Ferree, K., Allen, L.D., Kus, B.E., and Lynn, S., 2012, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2012 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Ferree, K., Allen, L.D., Kus, B.E., and Lynn, S., 2014, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2013 annual report: U. S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California.

44 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Ferree, K., Allen, L.D., Kus, B.E., and Pottinger, R., 2015, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2014 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California.

Houston, A., Allen, L.D., Pottinger, R.E., and Kus, B.E., 2021, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey flood risk management project area in San Diego County, California—Breeding activities and habitat use—2020 annual report: U.S. Geological Survey Open-File Report 2021–1053, 67 p. [Available at https://doi.org/10.3133/ofr20211053.]

Franzreb, K.E., 1989, Ecology and conservation of the endangered Least Bell's Vireo: U.S. Fish and Wildlife Service, Department of the Interior Biological Report, v. 89, no. 1, 17 p.

Houston, A., Allen, L.D., Pottinger, R.E., and Kus, B.E., 2022, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey flood risk management project area in San Diego County, California—Breeding activities and habitat use—2021 annual report: U.S. Geological Survey Open-File Report 2022–1012, 79 p. [Available at https://doi.org/10.3133/ofr20221012.]

Houston, A., Allen, L.D., Pottinger, R., and Kus, B.E., 2015, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2015 annual report: U.S Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Houston, A., Allen, L.D., Pottinger, R., and Kus, B.E., 2016, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2016 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Houston, A., Allen, L.D., Pottinger, R., Lynn, S., and Kus, B.E., 2017, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2017 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Houston, A., Allen, L.D., Pottinger, R., Lynn, S., and Kus, B.E., 2018, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2018 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California. Houston, A., Allen, L.D., Pottinger, R., Lynn, S., and Kus, B.E., 2019, Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2019 annual report: U.S. Geological Survey, prepared for RECON Environmental, Inc., San Diego, California.

Howell, S.L., and Kus, B.E., 2022, Distribution and abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Upper San Luis Rey River, San Diego County, California—2021 data summary: Data Report 1158, 11 p., accessed November 15, 2022, at https://doi.org/10.3133/dr1158. Kus, B.E., 1998, Use of restored riparian habitat by the endangered Least Bell’s Vireo (Vireo bellii pusillus): Restoration Ecology, v. 6, no. 1, p. 75–82. [Available at https://doi.org/10.1046/j.1526-100x.1998.06110.x.] Kus, B.E., 1999, Impacts of Brown-headed Cowbird parasitism on the productivity of the endangered Least Bell’s Vireo: Studies in Avian Biology, v. 18, p. 160–166. Kus, B.E., Hopp, S.L., Johnson, R.R., and Brown, B.T., 2020, Bell's Vireo (Vireo bellii), version 1.0, in Poole, A.F., ed., Birds of the World: Ithaca, N.Y., Cornell Lab of Ornithology, accessed October 20, 2022, at https://doi.org/10.2173/bow.belvir.01. Kus, B.E., and Rourke, J.W., 2005, Breeding activities of the Southwestern Willow Flycatcher in the San Luis Rey Flood Risk Management Project Area, San Diego County, California, in 2005: U.S. Geological Survey, prepared for RECON Environmental, San Diego, California. Kus, B.E., and Whitfield, M.J., 2005, Parasitism, productivity, and population growth—Response of Least Bell's Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) to cowbird (Molothrus spp.) control: Ornithological Monographs, no. 57, p. 16–27. [Available at https://pubs.er.usgs.gov/publication/1008117. Laake, J.L., 2013, RMark—An R Interface for analysis of capture-recapture data with MARK: Alaska Fisheries Science Center, Processed Report 2013-01, 25 p., accessed December 5, 2022, at https://repository.library.noaa.gov/ view/noaa/4372/noaa_4372_DS1.pdf.

References Cited 45 National Oceanic and Atmospheric Administration, 2022, Local climatological data: National Oceanic and Atmospheric Administration, accessed November 28, 2022, at https://www.ncei.noaa.gov/. R Core Team, 2021, The R Project for statistical computing: Vienna Austria, R Foundation for Statistical Computing, accessed November 28, 2022, at http://www.R-project.org. Remsen, J.V., Jr., 1978, Bird species of special concern in California: California Department of Fish and Game, Wildlife Management Division, Administrative Report 78-1. Riparian Habitat Joint Venture, 2004, The riparian bird conservation plan—A strategy for reversing the decline of riparian associated birds in California, version 2.0: California Partners in Flight. Schlorff, R.W., 1990, Status review of the Willow Flycatcher (Empidonax traillii) in California: Report to the Fish and Game Commission, State of California Resources Agency. Sedgwick, J.A., 2020, Willow Flycatcher (Empidonax traillii), version 1.0. in Poole, A.F., ed., Birds of the World: Ithaca, N.Y., Cornell Lab of Ornithology, accessed December 12, 2022, at https://doi.org/10.2173/bow.wilfly.01. Sogge, M.K., Ahlers, D., and Sferra, S.J., 2010, A natural history summary and survey protocol for the Southwestern Willow Flycatcher: U.S. Geological Survey Techniques and Methods book 2, chap. A10, 38 p. [Available at https://doi.org/10.3133/tm2A10.] Unitt, P., 1987, Empidonax traillii extimus—An endangered subspecies: Western Birds, v. 18, p. 137–162. U.S. Fish and Wildlife Service, 1986, Determination of endangered status for the Least Bell’s Vireo: Federal Register, v. 51, no. 85, p. 16474–16482.

U.S. Fish and Wildlife Service, 1993, Proposed rule to list the Southwestern Willow Flycatcher as endangered with critical habitat: Federal Register, v. 58, no. 140, p. 39495–39522. U.S. Fish and Wildlife Service, 1998, Draft recovery plan for the Least Bell’s Vireo (Vireo bellii pusillus): Portland, Oreg., U.S. Fish and Wildlife Service, 139 p. U.S. Fish and Wildlife Service, 2000, Southwestern Willow Flycatcher protocol revision: Sacramento, Calif., U.S. Fish and Wildlife Service, California/Nevada Operations Office. U.S. Fish and Wildlife Service, 2002, Southwestern Willow Flycatcher Recovery Plan: Albuquerque, N. Mex., U.S. Fish and Wildlife Service, 210 p. U.S. Fish and Wildlife Service, 2006a, Least Bell’s Vireo (Vireo bellii pusillus) 5-year review summary and evaluation: Carlsbad, Calif., U.S. Fish and Wildlife Service, Carlsbad Fish and Wildlife Office, 24 p. U.S. Fish and Wildlife Service, 2006b, Section 7 consultation and confirmation of a conference opinion on the operation and maintenance of the San Luis Rey River Flood Control Channel in the City of Oceanside, San Diego County, California (1-6-87-F-17R2). White, G.C., and Burnham, K.P., 1999, Program MARK— Survival estimation from populations of marked animals: Bird Study, v. 46, no. sup1, p. S120–S139. [Available at https://doi.org/10.1080/00063659909477239.] Whitfield, M.J., and Sogge, M.K., 1999, Range-wide impact of Brown-headed Cowbird parasitism on the Southwestern Willow Flycatcher (Empidonax traillii extimus): Studies in Avian Biology, v. 18, p. 182–190.

Giant reed removal

Vegetation removal

Herbicide treatment

Vegetation removal

Herbicide treatment

Vegetation removal

Giant reed removal and herbicide treatment

Herbicide treatment

Vegetation removal

March 2006

October 2006

November– December 2007

February– March 2008

July 2008

September– December 2008

September– October 2008

March 2009

October– November 2009

Vegetation treatment

December 2005

Date

Reaches 2–4

Treatment location

18.0 (44.5)

9.4 (23.2)

Area [ha (acres)]

Reach 4 (below College Boulevard)

0.4 (1.0)

Reach 1

Reach 1 (Benet Road to Interstate 5 and south of main river channel) 6.5 (16.1)

11.9 (29.4)

Reaches 2–4

Reach 1 (downstream of Benet Road)

3.2 (7.9)

2.3 (5.7)

37.9 (93.7)

0.08 (0.2)

Mowing with material handremoved.

Reach 1

5.1 (12.6)

2009 Least Bell’s Vireo breeding season (April–July)

Re-treatment of giant reed sprayed Reaches 3b and 4 in 2005.

Re-treatment of giant reed sprayed Reaches 2–4 in 2005.

First Phase 1 mowing.

Pepperweed sprayed.

2008 Least Bell’s Vireo breeding season (April–July)

First Phase 12 mowing, not completed.

Giant reed sprayed with herbicide.

2007 Least Bell’s Vireo breeding season (April–July)

Giant reed sprayed with herbicide.

2006 Least Bell’s Vireo breeding season (April–July)

Second phase of Risk Reduction was to mow/mulch vegetation 30 m wide.

First phase of Risk Reduction1 was to remove large stands of giant reed.

Vegetation management

[ha, hectare; —, no data; m, meter; km, kilometer; GPS, Global Positioning System]

Comments

—

Extensive mowing and chemical re-treatment of some giant reed areas that had been targeted in 2005 were done from Benet Road to College Boulevard. This completed the initial clearing of Phase 1.

Phase 1 mowing continued after vireo breeding season.

—

Only 7 days of mowing completed before rains and vireo breeding season began.

—

This spraying was an experimental effort to see if the herbicide after mowing approach would be effective.

30-m swath of vegetation in the river channel from Benet Road to College Boulevard (7.5 km) was mowed and mulched, overlapping when possible with the giant reed eradication areas.

Giant reed was mowed using a large mower with biomass chipped or shredded in situ.

Table 1.1. Timeline and description of vegetation treatments at the San Luis Rey Flood Risk Management Project Area, California, 2005–22.

Appendix 1. 46 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Herbicide treatment

Vegetation removal

Herbicide treatment

Vegetation removal

Herbicide treatment

Vegetation removal

Active restoration

Exotic vegetation removal

Pole cuttings

Vegetation removal

Active restoration

December 2010

February 2011

August– December 2011

September– October 2011

July– September 2012

October– November 2012

November– December 2012

October 2012

September 2013– January 2014

March–April 2014

October 2014

September– November 2014

Vegetation treatment

October– December 2010

Date

Treatment location

Area [ha (acres)]

Reach 1

8.2 (20.3)

Reaches 1, 2, and 4

Reaches 1–4

32.8 (81.1)

34.4 (85.0)

Reach 1

Reaches 1–4

Reach 1 (west of Benet Road, north side of the river channel)

Reaches 2–4

11.9 (29.4)

18.1 (44.7)

—3

Reaches 1, Reach 2, Reach 3a and Reach 3b

Reaches 1–4, Upper Pond, Whelan Mitigation

0.2 (0.5)

—3

Planting early March; Bi-monthly watering, herbicide treatment and weed control

Phase 1, Phase 2, Rotation 1

Reach 1

Reaches 1–4

11.9 (29.4)

Comments

—

No GPS data recorded so exact location and acreage affected is not available.

—

Phase 1 mowing by the City of Oceanside.

Benet Road to Interstate 5 and College Boulevard to 152 m downstream from Foussat Road were treated before stopping for the season. Lower Pond, Tuley Canyon, and Park Pond also treated.

Resprouted giant reed was sprayed with herbicide within the area of Reach 1 that was mowed in 2009.

Phase 1 mowing by the City of Oceanside. Mowing not completed because of rains.

—

56.0 (138.4) —

2014 Least Bell’s Vireo breeding season (April–July)

Exotic plant and tree removal throughout Project Area.

2013 Least Bell’s Vireo breeding season (April–July)

First Phase 24 mowing completed, all reaches.

Giant reed sprayed.

Giant reed sprayed in Phase 1 mowed area.

Remedial planting

5.0 (12.4)

8.8 (21.7)

2012 Least Bell’s Vireo breeding season (April–July)

Third Phase 1 mowing completed.

Weed eradication throughout Project Area, except annually mowed area.

Planting started.

Reach 1

Reaches 3a and 3b

2011 Least Bell’s Vireo breeding season (April–July)

Giant reed sprayed with herbicide.

Second Phase 1 mowing not completed.

2010 Least Bell’s Vireo breeding season (April–July)—Continued

Giant reed sprayed.

2010 Least Bell’s Vireo breeding season (April–July)

Vegetation management

[ha, hectare; —, no data; m, meter; km, kilometer; GPS, Global Positioning System]

Table 1.1. Timeline and description of vegetation treatments at the San Luis Rey Flood Risk Management Project Area, California, 2005–22.—Continued

Appendix 1. 47

Active restoration

Vegetation removal

Restoration enhancement

Herbicide treatment

Vegetation removal

Restoration enhancement

Vegetation removal

Herbicide spraying

March– November 2014

November– December 2015

December 2015– January 2016

October– November 2016

September– October 2017

November 2017

February 2018

Fall 2018

April 2018

Vegetation treatment

March– November 2014

Date

Reach 3a

Park Pond

Upper Pond

Reaches 1–4

Project-wide

0.1 (0.2)

4.0 (9.9)

2.5 (6.2)

18.3 (45.1)

Upper Pond and Park Pond

—3

28.2 (69.7)

Annual mowing Phase 1 and Phase 2

Upper Pond and Park Pond

Reach 2

—3

16.2 (40.1)

2018 Least Bell’s Vireo breeding season (April–July)

Giant reed cut and removed

Herbicide spraying

Reach 1, 3, 4

—

Comments

Herbicide applied to exotic weeds around pole plantings in Upper Pond and Park Pond. No GIS data collected to calculate area.

—

Giant reed was cut and removed from Upper Pond and Park Pond. No GIS data collected to calculate area.

Spraying and cutting of exotic weeds in Upper Pond and Park Pond. No GIS data collected to calculate area.

Boundary mowing occurred in Reach 1 in preparation for sediment removal.

Hand spraying of exotic weeds throughout Project Area.

—

65.8 (162.6) —

2017 Least Bell’s Vireo breeding season (April–July) Annual mowing Phase 1 and boundary mowing

Weed eradication

10.5 (25.9)

0.4 (1.0)

Area [ha (acres)]

2016 Least Bell’s Vireo breeding season (April–July)

Manual trimming and mowing

Annual mowing Phase 1, Phase 2, and Phase 3; Rotational mowing Rotation 1 and Rotation 2

Spraying

Whelan Mitigation

Reach 4

Treatment location

2015 Least Bell’s Vireo breeding season (April–July)

Earthwork March 6–April 20, 2014; Planting May–June 2014; Bi-weekly watering May– November 2014

Planting early March; Bi-monthly watering, herbicide treatment and weed control

Vegetation management

[ha, hectare; —, no data; m, meter; km, kilometer; GPS, Global Positioning System]

Table 1.1. Timeline and description of vegetation treatments at the San Luis Rey Flood Risk Management Project Area, California, 2005–22.—Continued

48 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Herbicide spraying

Vegetation removal

November– December 2020

November– December 2021

Treatment location

Area [ha (acres)]

Reaches 1–4

31.2 (77.2)

Reach 1 and Whelan restoration

—3

Annual mowing Phase 1 and Phase 2

Reaches 1–4

—

Spraying of exotic weeds in restoration areas. No GPS data recorded so exact location and acreage affected is not available.

—

Comments

4Phase 2 mowing includes mowing associated with maintaining the Phase 1 mowed area.

3Acreage not available.

2Phase 1 mowing was the first phase of the San Luis Rey Flood Risk Management Project to remove vegetation from the San Luis Rey River flood control channel to reduce the risk of flooding.

31.8 (78.5)

2021 Least Bell’s Vireo breeding season (April–July)

Herbicide Spraying

2020 Least Bell’s Vireo breeding season (April–July)

Annual mowing Phase 1 and Phase 2

2019 Least Bell’s Vireo breeding season (April–July)

Vegetation management

1Risk Reduction was implemented in 2005 to reduce the risk of flooding during the 2006 rainy season.

Vegetation removal

Vegetation treatment

November– December 2019

Date

[ha, hectare; —, no data; m, meter; km, kilometer; GPS, Global Positioning System]

Table 1.1. Timeline and description of vegetation treatments at the San Luis Rey Flood Risk Management Project Area, California, 2005–22.—Continued

Appendix 1. 49

50 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Appendix 2. Locations of Least Bell’s Vireo Territories and Completed Nests at the San Luis Rey Flood Risk Management Project Area, California, 2022 117°23'

117°22'

BMUL

NE TR

BTHR

BSAV BLAS

33° 13'

BSUS BDEE

BPAM

BPAR BT01 BGOO

BLUG BGRA BLEM BSHE BRAT BSWE BSAL BSHC BBOA BSOK BT02 BCAR BCHE BHAN BKEN BBUT BT03 BWAL BMAH BFIL BT04 BALM BPIS BTOK BCHS BPEA BSPE BCAS BHAZ

BPOI BORR BMRI

BPAT

BBEA

2022 Least Bell's Vireo (LBVI) territories and nests Survey sites: Reach 1, Lower Pond, and Tuley Canyon

TE TA

Restoration Survey site

# *

LBVI pair, not monitored LBVI unknown status, not monitored LBVI transient

5 0

225

450

900 Meters

Figure 2.1. Locations of Least Bell’s Vireo territories in the Reach 1, Lower Pond, and Tuley Canyon survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Appendix 2. 51 117°21'

WMIL FO

! ª WH27

! ( SA T RD

WH25

! (! (

FO40 ! (

! (

BLAS

FNER ! ª !

FBRI ! ª! (

FAQU ! ª

FO11

! (

FO6

! (

! ( ( FO2 !

! ª FO40

! (

FGOO

! (! (

( FO5 !

! (

(

BPAR BGOO

33° 13'

! (

( FRAS !

FO7

BGRA

2022 Least Bell's Vireo (LBVI) territories and nests Survey site: Reach 2 Restoration

! (

Successful nest

Survey site

! (

Unsuccessful nest

LBVI pair, fully monitored

! ª

Successful parasitized nest

LBVI pair, not monitored

! ª

Unsuccessful parasitized nest

LBVI unknown status, not monitored 0

125

250

500 Meters

Figure 2.2. Locations of Least Bell’s Vireo territories and nests in the Reach 2 survey site at the San Luis Rey Flood Risk Management Project Area, California, 2022.

52 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report 117°21'

117°20'

DCAL ! (

DMAC

CROD

( ! ( !

DMAD

ª (! ( (! !! (

DOUGLAS DR

WERY D157 DSIM ! DGAF ! ª ! ( ( DWIL ! ( (! DSAN DGAF ! ª DDOL ! ª ! ( ! WCAR ª ! ! ( ª ! ª ! ª ! ( ! ( ! WBON ! ( DMES ! ( ! ª( ( WSTA ! ! ª ! ª WEMI WKEL ! ª ! ª ( WGRI ! ! ( ! ª! ª! ª (WJUN ! ( ! ª! ( ! ( ! ª WEMM! WRAD WEAR

! (! (

CPAT ! ( ! (

CPAT

! (

! ( ( ! ! (

( WHEI WHEI! (! ª! W154 WDID ! ª ( ! ª! WH24! ( ª ! W155 ª WMAN ! WMAN WH29 ! ª

33° 14'

WFID

! ( ! (

! (

WH28 ! ª ! ( WH28

! ª! ª ª( ! WJEF ! ! ( WRUS ! (

WBAN ! (! ª

WH33 ( ! ª! ! (WH33 ! ª ! ª ! ª ( WDOB! ! ª WH20 WMIL FO US ! ( WH27 ! ( SA ! ª ! ( ( T ! WH25 R D

W158 W156

2022 Least Bell's Vireo (LBVI) territories and nests Survey sites: Reach 3a, Reach 3b, Park Pond, Pilgrim Pond, and Whelan Mitigation Restoration Survey site

! ( ! (

! ª LBVI pair, fully monitored ! ª LBVI pair, not monitored LBVI single male, fully monitored LBVI unknown status, not monitored

Successful nest Unsuccessful nest Successful parasitized nest Unsuccessful parasitized nest

175

350

700 Meters

Figure 2.3. Locations of Least Bell’s Vireo territories and nests in the Reach 3a, Reach 3b, Park Pond, Pilgrim Pond, and Whelan Mitigation survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Appendix 2. 53 117°19'

117°18'

33° 15'

CPAS

COL

! (

CEAS

LVD

CLAN

! ( ! (

CSOC CPOW

LEG

CWIL CPUR ! ( CPOW

( CSAT CIRO ! ! (

CBON ! (

! (

! ( ! (

CBOB ! ( CMYS

! ( CNED ! ( ! (

CSPK

CCHA

! ( CCHC

DOUGLAS DR

! (! (

CROD ! ( ! (

CPAT ! (CPAT

CHOO

! ( CMAC

( CLAD ! ( CSCR ! ! ( CBUT

! (

( CACD!

! (

C21P ( ( (CMET ! ( ! ! (! CJAS ! ! ( ! ( CBIL CJAS ( ! ( CBAN !

! ( CSCH CDIA ! ( ! ( CSNE CQTI CSTA CACA CSTR ! ( CCOT ! (

! (

( ! (!

CSAN

( ! (!

! ( ! ! (

CRED (

! ( ! ( ! (

2022 Least Bell's Vireo (LBVI) territories and nests Survey sites: Reach 4, Riverside Pond, and Upper Pond Restoration

LBVI pair, fully monitored

Survey site

LBVI unknown status, fully monitored

! (

Successful nest

LBVI unknown status, not monitored

! (

Unsuccessful nest

175

350

700 Meters

Figure 2.4. Locations of Least Bell’s Vireo territories and nests in the Reach 4, Riverside Pond, and Upper Pond survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

54 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Appendix 3. Table 3.1. Status and nesting activities of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022 [Nest fate: INC, nest never completed; OTH, reason for nest failure known, such as substrate failure; PAR, failed as a result of Brown-headed Cowbird parasitism; PRE, nest failure caused by predation event; SUC, fledged at least one Least Bell’s Vireo young; UNK, reason for nest failure/abandonment unknown. Abbreviations: —, no data; BHCO, Brown-headed Cowbird]

Territory

Nest

Nest fate

Number fledged

Reach 2

FAQU

PRE

—

Reach 2

FBRI

PRE

—

Reach 2

FO2

SUC

—

Reach 2

FO5

SUC

—

Reach 2

FO6

SUC

—

Reach 2

FNER

SUC

—

Reach 2

FRAS

SUC

—

Reach 2

FO7

PRE

—

Reach 2

FGOO

SUC

—

Reach 2

FO40

PAR

Failed with one BHCO egg.

Reach 2

FAQU

PRE

Removed one BHCO egg.

Reach 2

FBRI

SUC

Removed one BHCO egg.

Reach 2

FO2

PRE

—

Reach 2

FO5

UNK

Abandoned with eggs.

Reach 2

FO7

PRE

—

Reach 2

FO11

PRE

—

Reach 2

FO40

PRE

—

Reach 2

FNER

UNK

Removed one BHCO egg, abandoned.

Reach 2

FRAS

PRE

—

Reach 2

FO5

SUC

—

Reach 2

FO2

SUC

—

Reach 3a

WBON

PRE

Removed two BHCO eggs.

Reach 3a

WCAR

PRE

—

Reach 3a

WDID

PRE

Removed one BHCO egg.

Reach 3a

WJUN

INC

—

Reach 3a

WJUN

PRE

—

Reach 3a

WKEL

UNK

Eggs never confirmed.

Reach 3a

WH24

PRE

Removed one BHCO egg.

Reach 3a

WH25

PRE

—

Reach 3a

WH25

PRE

—

Reach 3a

WH27

SUC

—

Reach 3a

WH28

PRE

—

Reach 3a

WH33

UNK

Eggs never confirmed.

Reach 3a

WH33

PRE

Removed one BHCO egg.

Reach 3a

WDID

PRE

—

Reach 3a

WHEI

PAR

Removed two BHCO eggs, but nest failed prior to removal.

Reach 3a

WJUN

SUC

Removed one BHCO egg.

Survey site

Comments

Channel

Appendix 3. 55 Table 3.1. Status and nesting activities of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022—Continued [Nest fate: INC, nest never completed; OTH, reason for nest failure known, such as substrate failure; PAR, failed as a result of Brown-headed Cowbird parasitism; PRE, nest failure caused by predation event; SUC, fledged at least one Least Bell’s Vireo young; UNK, reason for nest failure/abandonment unknown. Abbreviations: —, no data; BHCO, Brown-headed Cowbird]

Survey site

Territory

Nest

Nest fate

Number fledged

Comments

Channel—Continued Reach 3a

WEMM

PRE

Removed one BHCO egg.

Reach 3a

WKEL

PRE

Removed one BHCO egg.

Reach 3a

WMAN

PAR

Removed two BHCO eggs, but nest failed prior to removal.

Reach 3a

WMAN

SUC

Removed one BHCO egg.

Reach 3a

WRAD

INC

—

Reach 3a

WRAD

UNK

Eggs never confirmed.

Reach 3a

WH20

PRE

Removed one BHCO egg.

Reach 3a

WH24

SUC

—

Reach 3a

WH25

SUC

—

Reach 3a

WH28

PRE

—

Reach 3a

WH33

SUC

—

Reach 3a

WRAD

UNK

Eggs never confirmed.

Reach 3a

WBON

PRE

Removed one BHCO egg.

Reach 3a

WKEL

SUC

Removed one BHCO egg.

Reach 3a

WRAD

SUC

—

Reach 3a

WHEI

SUC

—

Reach 3a

WDID

SUC

Removed one BHCO egg.

Reach 3a

WH28

SUC

Removed one BHCO egg.

Reach 3a

WH20

SUC

Removed one BHCO egg.

Reach 3a

WH27

SUC

Removed one BHCO egg.

Reach 3a

WEMM

UNK

Eggs never confirmed.

Reach 3a

WCAR

SUC

—

Reach 3a

WEAR

INC

—

Reach 3a

WEAR

UNK

Eggs never confirmed.

Reach 3a

WJUN

INC

—

Reach 3a

WJUN

UNK

Found after it had failed.

Reach 3b

DCAL

UNK

Eggs never confirmed.

Reach 3b

DGAF

PRE

—

Reach 3b

DMAD

INC

—

Reach 3b

DMES

INC

—

Reach 3b

DSIM

PRE

—

Reach 3b

DWIL

PRE

—

Reach 3b

DCAL

UNK

Eggs never confirmed.

Reach 3b

DCAL

SUC

—

Reach 3b

DDOL

PRE

Removed one BHCO egg.

Reach 3b

DMAD

UNK

Eggs never confirmed.

Reach 3b

DMES

PRE

—

Reach 3b

DMES

PAR

Failed with one BHCO egg.

Reach 3b

DSAN

PAR

Failed with two BHCO eggs.

56 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Table 3.1. Status and nesting activities of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022—Continued [Nest fate: INC, nest never completed; OTH, reason for nest failure known, such as substrate failure; PAR, failed as a result of Brown-headed Cowbird parasitism; PRE, nest failure caused by predation event; SUC, fledged at least one Least Bell’s Vireo young; UNK, reason for nest failure/abandonment unknown. Abbreviations: —, no data; BHCO, Brown-headed Cowbird]

Survey site

Territory

Nest

Nest fate

Number fledged

Comments

Channel—Continued—Continued Reach 3b

DSAN

UNK

Eggs never confirmed.

Reach 3b

DSIM

SUC

Removed two BHCO eggs.

Reach 3b

DWIL

PAR

Failed with two BHCO eggs.

Reach 3b

DGAF

PRE

—

Reach 3b

DMAD

UNK

Eggs never confirmed.

Reach 3b

DMAD

OTH

Egg laid and then removed by female.

Reach 3b

DMAD

OTH

Failed with one LBVI and one BHCO egg under the nest.

Reach 3b

DMES

PRE

Removed one BHCO egg.

Reach 3b

DSAN

PRE

Removed one BHCO egg.

Reach 3b

DWIL

SUC

—

Reach 3b

DMES

INC

—

Reach 4

CJAS

UNK

Eggs never confirmed.

Reach 4

CJAS

SUC

—

Reach 4

CPAT

INC

—

Reach 4

CACD

PRE

—

Reach 4

CBIL

SUC

—

Reach 4

CDIA

SUC

—

Reach 4

CMAC

INC

—

Reach 4

CROD

PRE

—

Reach 4

CROD

SUC

—

Reach 4

CSOC

SUC

—

Reach 4

C21P

UNK

Eggs never confirmed.

Reach 4

C21P

PRE

—

Reach 4

CBAN

SUC

—

Reach 4

CBON

PRE

—

Reach 4

CIRO

SUC

—

Reach 4

CLAD

INC

—

Reach 4

CMET

INC

—

Reach 4

CPAS

SUC

—

Reach 4

CPOW

PRE

—

Reach 4

CSAT

SUC

—

Reach 4

CSCH

OTH

Infertile eggs.

Reach 4

CJAS

PRE

—

Reach 4

CPAT

SUC

—

Reach 4

CACD

SUC

—

Reach 4

CBOB

PRE

—

Reach 4

CBOB

PRE

—

Reach 4

CCHA

PRE

—

Reach 4

CHOO

SUC

—

Appendix 3. 57 Table 3.1. Status and nesting activities of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022—Continued [Nest fate: INC, nest never completed; OTH, reason for nest failure known, such as substrate failure; PAR, failed as a result of Brown-headed Cowbird parasitism; PRE, nest failure caused by predation event; SUC, fledged at least one Least Bell’s Vireo young; UNK, reason for nest failure/abandonment unknown. Abbreviations: —, no data; BHCO, Brown-headed Cowbird]

Survey site

Territory

Nest

Nest fate

Number fledged

Comments

Channel—Continued—Continued—Continued Reach 4

CMET

SUC

—

Reach 4

CMYS

SUC

—

Reach 4

CNED

SUC

—

Reach 4

CPOW

PRE

—

Reach 4

CSAT

INC

—

Reach 4

CSCH

SUC

—

Reach 4

CSOC

SUC

—

Reach 4

CSPK

PRE

—

Reach 4

C21P

SUC

—

Reach 4

CCHC

SUC

—

Reach 4

CLAD

PRE

—

Reach 3a

WSTA

PAR

Failed with one BHCO egg.

Reach 3a

WSTA

SUC

Removed one BHCO egg.

Reach 3a

WDOB

UNK

Eggs never confirmed.

Reach 3a

WDOB

PRE

Removed one BHCO egg.

Reach 3a

WJEF

PAR

Failed with one BHCO egg.

Upper Pond

CPAT

PRE

—

Upper Pond

CSTA

INC

—

Upper Pond

CSTA

PRE

—

Upper Pond

CBUT

SUC

—

Upper Pond

CRED

PRE

—

Upper Pond

CSAN

INC

—

Upper Pond

CSNE

PRE

—

Upper Pond

CACA

PRE

—

Upper Pond

CQTI

SUC

—

Upper Pond

CSNE

SUC

—

Upper Pond

CACA

SUC

—

Upper Pond

CCOT

PRE

—

Upper Pond

CRED

SUC

—

Upper Pond

CSAN

SUC

—

Upper Pond

CSCR

SUC

—

Upper Pond

CSTA

PRE

—

Upper Pond

CSTA

SUC

—

Whelan Mitigation

WBAN

UNK

Eggs never confirmed but found ripped down.

Whelan Mitigation

WBAN

PRE

Removed one BHCO egg.

Whelan Mitigation

WFID

SUC

—

Whelan Mitigation

WJEF

PRE

Removed one BHCO egg.

Whelan Mitigation

WDOB

PRE

Removed one BHCO egg.

Off-channel

58 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Table 3.1. Status and nesting activities of Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022—Continued [Nest fate: INC, nest never completed; OTH, reason for nest failure known, such as substrate failure; PAR, failed as a result of Brown-headed Cowbird parasitism; PRE, nest failure caused by predation event; SUC, fledged at least one Least Bell’s Vireo young; UNK, reason for nest failure/abandonment unknown. Abbreviations: —, no data; BHCO, Brown-headed Cowbird]

Survey site

Territory

Nest

Nest fate

Number fledged

Comments

Off-channel—Continued Whelan Mitigation

WRUS

PRE

—

Whelan Mitigation

WRUS

OTH

Eggs never hatched.

Whelan Mitigation

WJEF

PRE

Removed one BHCO egg.

Whelan Mitigation

WFID

INC

—

Whelan Mitigation

WJEF

UNK

Found after it had failed.

Whelan Mitigation

WGRI

PRE

Removed one BHCO egg.

Whelan Mitigation

WGRI

PRE

Removed one BHCO egg.

Whelan Mitigation

WGRI

PAR

Removed one BHCO egg.

Restoration

Appendix 4. 59

Appendix 4. Table 4.1. Banded adult Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022 [Band combination: unique combination of colored leg bands: BKBK, plastic black; BKYE, plastic black-yellow split; BPST, plastic black-pink striped; BWST, plastic blue-white striped; BYST, plastic black-yellow striped; DBDP, plastic dark blue-dark pink split; DBWH, plastic dark blue-white split; DPDB, plastic dark pink-dark blue split; DPDP, plastic dark pink; DPWH, plastic dark pink-white split; gogo, metal gold; Mdb, dark blue numbered federal band; Mgo, gold numbered federal band; pupu, metal purple; PUPU, plastic purple; PUWH, plastic purple-white split; PUYE, plastic purple-yellow split; WHDB, plastic white-dark blue split; WHDP, plastic white-dark pink split; WHPU, plastic white-purple split; WHWH, plastic white; YEBK, plastic yellow-black split; YEPU, plastic yellow-purple split; YEYE, plastic yellow. Age: Exact age determined from uniquely numbered metal band observed during recapture; estimated age applies to an unbanded bird captured as an adult. Sex: M, male; F, female. Abbreviations: yr, year; ≥, greater than or equal to]

Band combination (left leg:right leg)

Age

Sex

Comments

BBOA

DBWH Mdb:PUYE

2 yrs

Banded as a nestling in FO12 territory in 2020. Color banded in BBOA territory in 2021.

BFIL

PUWH Mdb:PUYE

2 yrs

Banded as a nestling in FNER territory in 2020. Color banded in BFIL territory in 2021.

BKEN

YEBK:WHDP Mdb

3 yrs

Banded as a nestling in CJET territory in 2019. Color banded in DNIC territory in 2021.

BPAM

?:PUYE

≥1 yr

Banded at the San Luis Rey River before 2022.

BT02

pupu:PUWH Mdb

1 yr

Banded as a nestling in FO8 territory in 2021. Color banded in BT02 territory in 2022.

BT03

WHPU Mdb:PUYE

2 yrs

Banded as a nestling in FGOO territory in 2020. Color banded in BBUT territory in 2021.

BTOK

BYST Mdb:PUYE

2 yrs

Banded as a nestling in FGOO territory in 2020. Color banded in BTOK territory in 2021.

BTOK

BPST:Mdb

3 yrs

Banded as a nestling in WH25 territory in 2019. Color banded in BFIL territory in 2020.

CACA

BWST Mdb:WHPU

≥1 yr

Banded as an adult in CACA territory in 2022.

CBAN

PUWH Mdb:DBDP

≥4 yrs

Banded as an adult in CDIA territory in 2019.

CBIL

:Mdb

≥1 yr

Banded as a nestling at the San Luis Rey River before 2022.

CBON

DPDB Mdb:DPWH

≥1 yr

Banded as an adult in CBON territory in 2022.

CBUT

YEBK:BWST Mdb

≥4 yrs

Banded as an adult in CBUT territory in 2019.

CCHA

DPWH Mdb:pupu

≥1 yr

Banded as an adult in CCH territory in 2022.

CCOT

DBWH Mdb:

7 yrs

Banded as a nestling in CSCH territory in 2015. Color banded in CCOT territory in 2016.

CLAN

Mdb:

≥1 yr

Banded as a nestling at the San Luis Rey River before 2022.

CNED

WHDB Mdb:?

≥1 yr

Banded at the San Luis Rey River before 2022.

CPAS

WHDP Mdb:YEPU

≥3 yrs

Banded as an adult in CPAS territory in 2020.

CRED

DPDP:PUYE Mdb

3 yrs

Banded as a nestling in WOUT territory in 2019. Color banded in CTRO territory in 2020.

CROD

PUYE:WHDB Mdb

≥4 yrs

Banded as an adult in CSCR territory in 2019.

CSAT

DPDB pupu:Mdb

3 yrs

Banded as a nestling in CFLO territory in 2019. Color banded in CRED territory in 2021.

CSNE

pupu:BPST Mdb

5 yrs

Banded as a nestling in WKEL territory in 2017. Color banded in CACE territory in 2018.

CSOC

Mdb:BPST

2 yrs

Banded as a nestling in CNED territory in 2020. Color banded in CPOW territory in 2021.

CWIL

DBDP Mdb:PUWH

≥4 yrs

Banded as an adult in CSOC territory in 2019.

DCAL

BPST Mdb:gogo

≥3 yrs

Banded as an adult in DBOW territory in 2020.

DDOL

:Mdb

≥1 yr

Banded as a nestling at the San Luis Rey River before 2022.

Territory

60 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Table 4.1. Banded adult Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022—Continued [Band combination: unique combination of colored leg bands: BKBK, plastic black; BKYE, plastic black-yellow split; BPST, plastic black-pink striped; BWST, plastic blue-white striped; BYST, plastic black-yellow striped; DBDP, plastic dark blue-dark pink split; DBWH, plastic dark blue-white split; DPDB, plastic dark pink-dark blue split; DPDP, plastic dark pink; DPWH, plastic dark pink-white split; gogo, metal gold; Mdb, dark blue numbered federal band; Mgo, gold numbered federal band; pupu, metal purple; PUPU, plastic purple; PUWH, plastic purple-white split; PUYE, plastic purple-yellow split; WHDB, plastic white-dark blue split; WHDP, plastic white-dark pink split; WHPU, plastic white-purple split; WHWH, plastic white; YEBK, plastic yellow-black split; YEPU, plastic yellow-purple split; YEYE, plastic yellow. Age: Exact age determined from uniquely numbered metal band observed during recapture; estimated age applies to an unbanded bird captured as an adult. Sex: M, male; F, female. Abbreviations: yr, year; ≥, greater than or equal to]

Territory

Band combination (left leg:right leg)

Age

Sex

Comments

DMAC

BYST Mdb:PUWH

≥3 yrs

Banded as an adult in DGAF territory in 2020.

DMES

pupu:BKBK Mdb

≥3 yrs

Banded as an adult in WSHA territory in 2020.

DSIM

DBDP:YEBK Mdb

≥3 yrs

Banded as an adult in WEAR territory in 2020.

DWIL

YEPU Mdb:WHDB

6 yrs

Banded as a nestling in BSAV territory in 2016. Color banded in DWIL territory in 2017.

FNER

DBDP Mdb:gogo

≥4 yrs

Banded as an adult in FNER territory in 2019.

FO11

Mgo:

≥1 yr

Banded as a nestling at the Santa Margarita River (MCBCP) before 2022.

FO2

BKBK Mdb:YEBK

2 yrs

Banded as a nestling in DCHE territory in 2020. Color banded in FO2 territory in 2021.

FO40

Mdb:

≥1 yr

Banded as a nestling at the San Luis Rey River before 2022.

FO5

BKYE:DPDP Mdb

5 yrs

Banded as a nestling in BGRA territory in 2017. Color banded in FO1 territory in 2018.

FO6

BWST Mdb:BPST

≥2 yrs

Banded as an adult in FO4 territory in 2021.

FO7

DBDP Mdb:BPST

≥2 yrs

Banded as an adult in FO7 territory in 2021.

W154

pupu:DPDB Mdb

3 yrs

Banded as a nestling in DDOL territory in 2019. Color banded in W153 territory in 2020.

W155

YEPU:BKYE Mdb

5 yrs

Banded as a nestling in FNER territory in 2017. Color banded in WTHA territory in 2018.

W155

:Mdb

≥1 yr

Banded as a nestling at the San Luis Rey River before 2022.

WBAN

DPDB Mdb:DBDP

2 yrs

Banded as a nestling in CCOT territory in 2020. Color banded in WH33 territory in 2021.

WCAR

WHDB pupu:Mdb

≥1 yr

Banded as an adult in WCAR territory in 2022.

WDOB

:Mgo

≥1 yr

Banded as a nestling at the Santa Margarita River (MCBCP) before 2022.

WEAR

DPDB:PUWH Mdb

≥1 yr

Banded as an adult in WEAR territory in 2022.

WEMI

BWST pupu:Mdb

≥1 yr

Banded as an adult in WEMI territory in 2022.

WFID

DPWH:BPST Mdb

≥4 yrs

Banded as an adult in WFID territory in 2019.

WFID

DPWH Mdb:WHDP

≥5 yrs

Banded as an adult in WRAD territory in 2018.

WGAF

DPDB:YEYE Mdb

3 yrs

Banded as a nestling in WGAR territory in 2019. Color banded in WGAF territory in 2021.

WH20

BWST:BKYE Mdb

≥1 yr

Banded as an adult in WH20 territory in 2022.

WH20

:Mdb

≥1 yr

Banded as a nestling at the San Luis Rey River before 2022.

WH24

PUWH Mdb:YEBK

≥4 yrs

Banded as an adult in CFOR territory in 2019.

WH25

DPDB Mdb:WHPU

4 yrs

Banded as a nestling in DBEL territory in 2018. Color banded in WH25 territory in 2019.

WH28

DBWH Mdb:BKYE

≥3 yrs

Banded as an adult in WH01 territory in 2020.

WH29

DBDP:WHWH Mdb

3 yrs

Banded as a nestling in CCHA territory in 2019. Color banded in WH29 territory in 2020.

WJEF

PUWH Mdb:WHDP

2 yrs

Banded as a nestling in DSAN territory in 2020. Color banded in WH22 territory in 2021.

WJUN

BPST Mdb:PUPU

≥3 yrs

Banded as an adult in WKEL territory in 2020.

Appendix 4. 61 Table 4.1. Banded adult Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, California, 2022—Continued [Band combination: unique combination of colored leg bands: BKBK, plastic black; BKYE, plastic black-yellow split; BPST, plastic black-pink striped; BWST, plastic blue-white striped; BYST, plastic black-yellow striped; DBDP, plastic dark blue-dark pink split; DBWH, plastic dark blue-white split; DPDB, plastic dark pink-dark blue split; DPDP, plastic dark pink; DPWH, plastic dark pink-white split; gogo, metal gold; Mdb, dark blue numbered federal band; Mgo, gold numbered federal band; pupu, metal purple; PUPU, plastic purple; PUWH, plastic purple-white split; PUYE, plastic purple-yellow split; WHDB, plastic white-dark blue split; WHDP, plastic white-dark pink split; WHPU, plastic white-purple split; WHWH, plastic white; YEBK, plastic yellow-black split; YEPU, plastic yellow-purple split; YEYE, plastic yellow. Age: Exact age determined from uniquely numbered metal band observed during recapture; estimated age applies to an unbanded bird captured as an adult. Sex: M, male; F, female. Abbreviations: yr, year; ≥, greater than or equal to]

Territory

Band combination (left leg:right leg)

Age

Sex

Comments

WKEL

BPST Mdb:DBDP

≥3 yrs

Banded as an adult in DBRU territory in 2020.

WMAN

YEBK:DBDP Mdb

5 yrs

Banded as a nestling in BMAL territory in 2017. Color banded in WMAN territory in 2021.

WMIL

Mdb:DPDB

≥1 yr

Banded as an adult in WMIL territory in 2022.

WRAD

YEBK:WHPU Mdb

3 yrs

Banded as a nestling in FO4 territory in 2019. Color banded in WANI territory in 2021.

WRAD

:Mdb

≥1 yr

Banded as a nestling at the San Luis Rey River before 2022.

WSTA

PUWH Mdb:PUWH

≥3 yrs

Banded as an adult in WEMI territory in 2020.

62 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Appendix 5. Table 5.1. Between-year movement of emigrant Least Bell’s Vireos from the San Luis Rey Flood Risk Management Project Area to other areas outside of the Project Area, California, 2022. [Location: MCBCP, Marine Corps Base Camp Pendleton; FNWS, Fallbrook Naval Weapons Station. Band combination: unique combination of colored leg bands: BKBK, plastic black; BPST, plastic black-pink striped; DPDB, plastic dark pink-dark blue split; gogo, metal gold; Mdb, dark blue numbered federal band; pupu, metal purple; WHWH, plastic white; YEYE, plastic yellow. Age: Exact age determined from uniquely numbered metal band observed during recapture; estimated age applies to an unbanded bird captured as an adult; Sex: M, male; F, female. Abbreviations: km, kilometer, yr, year]

Drainage/natal territory

2022 Drainage/territory/location

San Luis Rey River/WTHA

Aliso Creek/AL02/MCBCP

San Luis Rey River/CBOB San Luis Rey River/WDID San Luis Rey River/CQTI

Dispersal distance (km)

Band combination (left leg:right leg)

Age in 2022

Sex F

9.84

BPST Mdb:BKBK

4 yrs

Santa Margarita River/AE02/MCBCP

8.65

gogo:WHWH Mdb

3 yrs

Fallbrook Creek/6SA-02/FNWS

15.30

pupu:YEYE Mdb

1 yr

Middle San Luis Rey River/MSL108

2.66

DPDB:Mdb

1 yr

Appendix 6. 63

Appendix 6. Table 6.1. Between-year movement of Least Bell’s Vireos banded as juveniles in 2021 at the San Luis Flood Risk Project Area, redetected in 2022. [Location: FNWS, Fallbrook Naval Weapons Station. Band combination: unique combination of colored leg bands; DPDB, plastic dark pink-dark blue split; Mdb, dark blue numbered federal band; pupu, metal purple; PUWH, plastic purple-white; YEYE, plastic yellow. Sex: M, male; F, female. Abbreviation: km, kilometer]

2021 Drainage/natal territory

2022 Drainage/territory

Dispersal distance (km)

Band combination (left leg:right leg)

Sex

San Luis Rey River/FO8

San Luis Rey River/BT02

2.28

pupu:PUWH Mdb

San Luis Rey River/WDID

Fallbrook Creek/6SA-02/FNWS

15.30

pupu:YEYE Mdb

San Luis Rey River/CQTI

Middle San Luis Rey River/MSL108

2.66

DPDB:Mdb

64 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Appendix 7. Table 7.1. Between-year movement of adult Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, 2022. [Year last detected for all detections was 2021. Drainage Codes: SLR, San Luis Rey River. Band combination: unique combination of colored leg bands; BKBK, plastic black; BKYE, plastic black-yellow split; BPST, plastic black-pink striped; BWST, plastic blue-white striped; BYST, plastic black-yellow striped; DBDP, plastic dark blue-dark pink split; DBWH, plastic dark blue-white split; DPDB, plastic dark-pink dark-blue split; DPDP, plastic dark pink; DPWH, plastic dark pink-white split; gogo, metal gold; Mdb, dark blue numbered federal band; PUPU, plastic purple; pupu, metal purple; PUWH, plastic purple-white split; PUYE, plastic purple-yellow split; WHDB, plastic white-dark blue split; WHDP, plastic white-dark pink split; WHPU, plastic white-purple split; WHWH, plastic white; YEBK, plastic yellow-black split; YEPU, plastic yellow-purple split; YEYE, plastic yellow; Age: Exact age determined from uniquely numbered metal band observed during recapture; estimated age applies to an unbanded bird captured as an adult; Sex: M, male; F, female. Abbreviations: km, kilometer; yrs, years; ≥, greater than or equal to]

Return to territory (within 0.1 km)

Last year detected drainage/territory

2022 Drainage/territory

Distance moved (km)

Band combination (left leg:right leg)

Age in 2022

Sex

Yes

SLR/WH29

0.00

DBDP:WHWH Mdb

3 yrs

Yes

SLR/CCOT

0.00

DBWH Mdb:

7 yrs

Yes

SLR/W154

0.00

pupu:DPDB Mdb

3 yrs

Yes

SLR/CRED

0.01

DPDP:PUYE Mdb

3 yrs

Yes

SLR/W155

0.01

YEPU:BKYE Mdb

5 yrs

Yes

SLR/CPAS

0.01

WHDP Mdb:YEPU

≥3 yrs

Yes

SLR/CROD

0.01

PUYE:WHDB Mdb

≥4 yrs

Yes

SLR/CWIL

0.01

DBDP Mdb:PUWH

≥4 yrs

Yes

SLR/DBOW

SLR/DCAL

0.01

BPST Mdb:gogo

≥3 yrs

Yes

SLR/FNER

0.01

DBDP Mdb:gogo

≥4 yrs

Yes

SLR/WH22

SLR/WJEF

0.02

PUWH Mdb:WHDP

2 yrs

Yes

SLR/WFID

0.02

DPWH:BPST Mdb

≥4 yrs

Yes

SLR/BFIL

0.02

PUWH Mdb:PUYE

2 yrs

Yes

SLR/WANI

SLR/WRAD

0.02

YEBK:WHPU Mdb

3 yrs

Yes

SLR/DWIL

0.02

YEPU Mdb:WHDB

6 yrs

Yes

SLR/BBUT

SLR/BT03

0.02

WHPU Mdb:PUYE

2 yrs

Yes

SLR/FO4

SLR/FO6

0.02

BWST Mdb:BPST

≥2 yrs

Yes

SLR/WGAF

0.02

DPDB:YEYE Mdb

3 yrs

Yes

SLR/CBAN

0.03

PUWH Mdb:DBDP

≥4 yrs

Yes

SLR/BBOA

0.03

DBWH Mdb:PUYE

2 yrs

Yes

SLR/FO7

0.04

DBDP Mdb:BPST

≥2 yrs

Yes

SLR/FO2

0.04

BKBK Mdb:YEBK

2 yrs

Yes

SLR/DMES

0.05

pupu:BKBK Mdb

≥3 yrs

Yes

SLR/WEMI

SLR/WSTA

0.06

PUWH Mdb:PUWH

≥3 yrs

Yes

SLR/CSCR

SLR/CBUT

0.08

YEBK:BWST Mdb

≥4 yrs

Yes

SLR/WH33

SLR/WBAN

0.09

DPDB Mdb:DBDP

2 yrs

Yes

SLR/WMAN

0.09

YEBK:DBDP Mdb

5 yrs

SLR/FGOO

SLR/FO5

0.12

BKYE:DPDP Mdb

5 yrs

SLR/BTOK

0.14

BYST Mdb:PUYE

2 yrs

SLR/WH21

SLR/WH25

0.14

DPDB Mdb:WHPU

4 yrs

SLR/CPOW

SLR/CSOC

0.15

Mdb:BPST

2 yrs

SLR/CSTR

SLR/CSNE

0.20

pupu:BPST Mdb

5 yrs

SLR/WJEF

SLR/WFID

0.26

DPWH Mdb:WHDP

≥5 yrs

SLR/WH26

SLR/WH28

0.31

DBWH Mdb:BKYE

≥3 yrs

Appendix 7. 65 Table 7.1. Between-year movement of adult Least Bell’s Vireos at the San Luis Rey Flood Risk Management Project Area, 2022.— Continued [Year last detected for all detections was 2021. Drainage Codes: SLR, San Luis Rey River. Band combination: unique combination of colored leg bands; BKBK, plastic black; BKYE, plastic black-yellow split; BPST, plastic black-pink striped; BWST, plastic blue-white striped; BYST, plastic black-yellow striped; DBDP, plastic dark blue-dark pink split; DBWH, plastic dark blue-white split; DPDB, plastic dark-pink dark-blue split; DPDP, plastic dark pink; DPWH, plastic dark pink-white split; gogo, metal gold; Mdb, dark blue numbered federal band; PUPU, plastic purple; pupu, metal purple; PUWH, plastic purple-white split; PUYE, plastic purple-yellow split; WHDB, plastic white-dark blue split; WHDP, plastic white-dark pink split; WHPU, plastic white-purple split; WHWH, plastic white; YEBK, plastic yellow-black split; YEPU, plastic yellow-purple split; YEYE, plastic yellow; Age: Exact age determined from uniquely numbered metal band observed during recapture; estimated age applies to an unbanded bird captured as an adult; Sex: M, male; F, female. Abbreviations: km, kilometer; yrs, years; ≥, greater than or equal to]

Return to territory (within 0.1 km)

Last year detected drainage/territory

2022 Drainage/territory

Distance moved (km)

Band combination (left leg:right leg)

Age in 2022

Sex

SLR/DO01

SLR/DMAC

0.52

BYST Mdb:PUWH

≥3 yrs

SLR/WANI

SLR/DSIM

0.78

DBDP:YEBK Mdb

≥3 yrs

SLR/WFID

SLR/WJUN

0.79

BPST Mdb:PUPU

≥3 yrs

SLR/DBRU

SLR/WKEL

1.00

BPST Mdb:DBDP

≥3 yrs

SLR/CRED

SLR/CSAT

1.06

DPDB pupu:Mdb

3 yrs

SLR/DGWE

SLR/WH24

2.03

PUWH Mdb:YEBK

≥4 yrs

SLR/DNIC

SLR/BKEN

4.03

YEBK:WHDP Mdb

3 yrs

66 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Appendix 8. Locations of Transient Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area, 2022 117°23'

117°22'

BT01F

_ ^

33° 13'

BT03F

_ ^

2022 Willow Flycatcher locations Survey sites: Reach 1, Lower Pond, and Tuley Canyon Survey site

_ ^

Transient Willow Flycatcher 0

175

350

700 Meters

Figure 8.1. Locations of transient Willow Flycatchers in Reach 1, Lower Pond, and Tuley Canyon survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Appendix 8. 67 117°21'

117°20'

DO01F DOUGLAS DR

_ ^ WH01F

_ ^

33° 14'

2022 Willow Flycatcher locations Survey sites: Reach 3a, Reach 3b, Park Pond, Pilgrim Pond, and Whelan Mitigation Survey site

SA T

_ ^ RD

Transient Willow Flycatcher 0

175

350

700 Meters

Figure 8.2. Locations of transient Willow Flycatchers in Reach 3a, Reach 3b, Park Pond, Pilgrim Pond, and Whelan Mitigation survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

68 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Appendix 9. Vegetation Transects, Nest-Centered Vegetation Plots, and Vegetation Treatments at the San Luis Rey Flood Risk Management Project Area, 2022 117°23'

117°22'

T NE BE RD

33° 13'

2022 vegetation study Survey sites: Reach 1, Lower Pond, and Tuley Canyon Vegetation removal 2005–2021 Manual trimming

Restoration 2011–2015

TE TA

Adaptive habitat management area Least Bell’s Vireo territory Survey site

200

400

800 Meters

Figure 9.1. Locations of vegetation treatments in Reach 1, Lower Pond, and Tuley Canyon survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Appendix 9. 69 117°21'

# * # *

FO 62 70 73

# * * # * # # *

# *# *

# *

#* * #

# * * ## # *# * *

SA T

# RD * # *

# *

# #* *

T NE BE RD 2022 vegetation study Survey site: Reach 2 Vegetation removal 2005–2021

Vegetation sampling points

Manual trimming

33° 13'

Channel

Restoration 2011–2015 Adaptive habitat management area

# * # *

Nest plot Territory plot

Least Bell’s Vireo territory Survey site 0

125

250

500 Meters

Figure 9.2. Locations of vegetation transects and territory plots in the Reach 2 survey site at the San Luis Rey Flood Risk Management Project Area, California, 2022.

70 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report 117°21'

117°20'

# * * # 44

WH-2 WH-1 46 48 33° 14'

* ## ## * * ** # *# # *# *

# * # *# * * #

* ## *

# * # *

DOUGLAS DR

B26

* ## * # * # *

*# 50 # * * ## *

2022 vegetation study Survey sites: Reach 3a, Reach 3b, Park Pond, Pilgrim Pond, and Whelan Mitigation

Vegetation removal 2005–2021

# *# * # * # *

SA T 62 RD

* ## *

Vegetation sampling points

Manual trimming

Channel

Restoration 2011–2015

Off-channel

Adaptive habitat management area Survey site 0

200

Restoration

# * # *

Least Bell’s Vireo territory 400

Nest plot Territory plot 800 Meters

* Base map from Esri and its licensors,# copyright 2022 World Geodetic System of 1984 Figure 9.3. Locations of vegetation transects and territory plots in the Reach 3a, Reach 3b, Park Pond, Pilgrim Pond, and Whelan Mitigation survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

Appendix 9. 71 117°19'

117°18'

COL

33° 15'

* # *#

LEG

LVD

2 4 6 8 10

DOUGLAS DR

# * # * # * # *

* # *# ## * *

* *# # * # *# # * # ## * # ** * * #* # # * *# # * B13 # * *# ## * * 2022 vegetation study B14 B22 B21 B20 B19 B18 B17 B15

* # # # * # * * *# # *# * *# * B26 B25 B24 B23 # *#

Survey sites: Reach 4, Riverside Pond, and Upper Pond Vegetation removal 2005–2021

Vegetation sampling points

Manual trimming

Channel

Restoration 2011–2015

Off-channel

Adaptive habitat management area 33° 14'

Least Bell’s Vireo territory

# * # *

Nest plot Territory plot

Survey site 0

200

400

800 Meters

Figure 9.4. Locations of vegetation transects and territory plots in Reach 4, Riverside Pond, and Upper Pond survey sites at the San Luis Rey Flood Risk Management Project Area, California, 2022.

72 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report

Appendix 10. Table 10.1. Global Positioning System coordinates (decimal degrees; World Geographic System of 1984 [WGS 84]) for the start and end points (quadrat) of each vegetation transect sampled at the San Luis Rey Flood Risk Management Project Area, 2006–22. [Quad indicates the distance in meters along a transect. Abbreviations: ID, identification; =, equals; —, not applicable]

Transect ID

Quad

X-West

Transect bearing

Y-North Reach 4

−117.29912

33.24834

Bearing=304 degrees

145

−117.30033

33.24907

—

−117.29964

33.24763

Bearing=310 degrees

115

−117.30074

33.24805

—

−117.30067

33.24607

Bearing=300 degrees

105

−117.30170

33.24649

—

−117.30167

33.24445

Bearing=300 degrees

105

−117.30260

33.24490

—

−117.30274

33.24299

Bearing=314 degrees

105

−117.30349

33.24361

—

−117.30434

33.24167

Bearing=330 degrees

115

−117.30506

33.24246

—

−117.30612

33.24078

Bearing=330 degrees

−117.30656

33.24148

—

−117.30823

33.24023

Bearing=352 degrees

115

−117.30851

33.24111

—

−117.31030

33.24005

Bearing=358 degrees

115

−117.31056

33.24110

—

−117.31255

33.23992

Bearing=358 degrees

115

−117.31248

33.24111

—

−117.31473

33.24009

Bearing=2 degrees

115

−117.31476

33.24105

—

−117.31675

33.23999

Bearing=2 degrees

105

−117.31678

33.24098

—

−117.31910

33.24006

Bearing=2 degrees

105

−117.31904

33.24093

—

−117.32116

33.24006

Bearing=2 degrees

115

−117.32106

33.24105

—

−117.32325

33.23991

Bearing=0 degrees

105

−117.32339

33.24101

—

−117.32537

33.24037

Bearing=0 degrees

115

−117.32544

33.24099

—

−117.32756

33.24004

Bearing=0 degrees

105

−117.32753

33.24099

—

−117.32965

33.24000

Bearing=0 degrees

105

−117.32968

33.24073

—

Reach 3b

Appendix 10. 73 Table 10.1. Global Positioning System coordinates (decimal degrees; World Geographic System of 1984 [WGS 84]) for the start and end points (quadrat) of each vegetation transect sampled at the San Luis Rey Flood Risk Management Project Area, 2006–22.—Continued [Quad indicates the distance in meters along a transect. Abbreviations: ID, identification; =, equals; —, not applicable]

Transect ID

Quad

X-West

Transect bearing

Y-North Reach 3b—Continued

−117.33178

33.23997

Bearing=0 degrees

115

−117.33180

33.24090

—

Reach 3a 40

−117.33602

33.23986

Bearing=0 degrees

−117.33606

33.24068

—

−117.33802

33.23924

Bearing=332 degrees

115

−117.33872

33.24006

—

−117.33963

33.23798

Bearing=296 degrees

105

−117.34059

33.23835

—

−117.34036

33.23622

Bearing=278 degrees

115

−117.34148

33.23634

—

−117.34067

33.23450

Bearing=284 degrees

165

−117.34229

33.23481

—

−117.34127

33.23273

Bearing=286 degrees

145

−117.34264

33.23309

—

−117.34311

33.22950

Bearing=286 degrees

135

−117.34445

33.22993

—

Reach 2 62

−117.34478

33.22253

Bearing=304 degrees

125

−117.34586

33.22306

—

−117.34641

33.22144

Bearing=326 degrees

145

−117.34730

33.22255

—

−117.34839

33.22074

Bearing=346 degrees

135

−117.34877

33.22220

—

−117.35272

33.22096

Bearing=6 degrees

145

−117.35252

33.22219

—

−117.35612

33.22054

Bearing=347 degrees

135

−117.35654

33.22166

—

B13

−117.30713

33.24002

Bearing=172 degrees

B13

−117.30683

33.23959

—

B14

−117.30823

33.23991

Bearing=172 degrees

B14

115

−117.30807

33.23901

—

B15

−117.30930

33.23978

Bearing=172 degrees

B15

115

−117.30912

33.23880

—

B17

−117.31148

33.23974

Bearing=172 degrees

B17

105

−117.31135

33.23885

—

B18

−117.31256

33.23971

Bearing=182 degrees

B18

105

−117.31270

33.23883

—

B19

−117.31372

33.23972

Bearing=182 degrees

Upper Pond

74 Least Bell’s Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report Table 10.1. Global Positioning System coordinates (decimal degrees; World Geographic System of 1984 [WGS 84]) for the start and end points (quadrat) of each vegetation transect sampled at the San Luis Rey Flood Risk Management Project Area, 2006–22.—Continued [Quad indicates the distance in meters along a transect. Abbreviations: ID, identification; =, equals; —, not applicable]

Transect ID

Quad

X-West

Transect bearing

Y-North Upper Pond—Continued

B19

105

−117.31391

33.23874

—

B20

−117.31475

33.23970

Bearing=182 degrees

B20

105

−117.31495

33.23881

—

B21

−117.31580

33.23972

Bearing=182 degrees

B21

105

−117.31602

33.23880

—

B22

−117.31689

33.23969

Bearing=182 degrees

B22

−117.31699

33.23885

—

B23

−117.31802

33.23968

Bearing=182 degrees

B23

−117.31800

33.23920

—

B24

−117.31901

33.23964

Bearing=182 degrees

B24

−117.31902

33.23930

—

B25

−117.32011

33.23963

Bearing=182 degrees

B25

−117.32010

33.23927

—

B26

−117.32110

33.23955

Bearing=182 degrees

B26

−117.32118

33.23925

—

Whelan Mitigation WH-1

−117.33969

33.23975

Bearing=345 degrees

WH-1

325

−117.34222

33.23774

—

WH-2

−117.33989

33.24004

Bearing=345 degrees

WH-2

345

−117.34281

33.23811

—

For more information concerning the research in this report, contact the Director, Western Ecological Research Center U.S. Geological Survey 3020 State University Drive East Sacramento, California 95819 https://www.usgs.gov/centers/werc Publishing support provided by the U.S. Geological Survey Science Publishing Network, Sacramento Publishing Service Center

Houston and others—Least Bell's Vireos and Southwestern Willow Flycatchers: Breeding Activities and Habitat Use—2022 annual report—OFR 2023–1040

ISSN 2331-1258 (online) https://doi.org/10.3133/ofr20231040

Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California: Breeding Activities and Habitat Use—2022 Annual Report

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