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Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 Annual Report

Scarlett L. Howell, Barbara E. Kus, M.R. Browning, K.P. Burnham, D.R. Anderson, D. Gaines · U.S. Geological Survey

USGS Publications · Papers · License: Public Domain

abundance activities annual base breeding california camp corps

usgs, geological survey, united states, science, government publication, usgs open-file report, 2024, 1039, distribution, abundance, breeding, activities, southwestern, willow, flycatcher, marine, corps, base, camp, pendleton, california, 2021, annual

Ecosystems Mission Area—Species Management Research Program Prepared in cooperation with Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton

Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 Annual Report

Open-File Report 2024–1039

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

Cover. Southwestern Willow Flycatcher (Empidonax traillii extimus) nest at Marine Corps Base Camp Pendleton. Photograph by S.L. Howell, U.S. Geological Survey, June 21, 2021.

Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 Annual Report By Scarlett L. Howell and Barbara E. Kus

Ecosystems Mission Area—Species Management Research Program Prepared in cooperation with Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton

Open-File Report 2024–1039

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

U.S. Geological Survey, Reston, Virginia: 2024

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: Howell, S.L., and Kus, B.E., 2024, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 annual report: U.S. Geological Survey Open-File Report 2024–1039, 35 p., https://doi.org/10.3133/ofr20241039. ISSN 2331-1258 (online)

iii

Acknowledgments This work was funded by the Environmental Security Department, Resources Management Division, Marine Corps Base Camp Pendleton, California. Data either are not available or have limited availability owing to restrictions of the funding entity (U.S. Marine Corps). Contact Ryan Besser ([email protected]) for more information. All activities were authorized by federal 10(a)1(A) Recovery Permit ESPER0004080_0. Parts of this report were written following a previously developed template to maintain consistent presentation of results. The authors thank the biologists who assisted in the data collection for this project: Lisa Allen, Armand Amico, Annabelle Bernabe, Christopher Aaron Gallagher, Alexandra Houston, Suellen Lynn, Rachelle McLaughlin, Shannon Mendia, Devin Taylor, and Michelle Treadwell (U.S. Geological Survey).

Contents Acknowledgments��iii Executive Summary��1 Introduction��1 Study Areas and Methods��3 Population Size and Distribution��3 Core Areas��3 Rotating Non-Core Areas: C��3 Habitat Characteristics��5 Conspecific Playback Survey Methods��5 Artificial Seep Monitoring��7 Breeding Productivity Methods��7 Nest Site Characteristics��8 Survivorship, Fidelity, and Movement��8 Annual Survivorship��8 Site Fidelity and Movement��9 Data Comparisons��9 Results��9 Population Size and Distribution��9 Transients��9 Residents��9 Conspecific Playback Surveys��11 Habitat Characteristics��11 Breeding Productivity��11 Nest Site Characteristics��11 Cowbird Parasitism��11 Survivorship, Site Fidelity, and Movement��15 Overview of Banded Population��15 Annual Survivorship��15 Survivorship Models—Adults Only ...............................................................................15 Survivorship Models—Adults Versus First-Year Flycatchers ...................................15 Site Fidelity and Movement��17 Discussion��17 Conclusions��19 References Cited��21 Appendix 1. Southwestern Willow Flycatcher Survey Areas at Marine Corps Base Camp Pendleton, California, 2021��24 Appendix 2. Locations of Willow Flycatchers at Marine Corps Base Camp Pendleton, California, 2021��30 Appendix 3. Southwestern Willow Flycatcher Territory Locations at Marine Corps Base Camp Pendleton, California, 2021��35

Figures 1. 2.

3. 4.

Map showing Southwestern Willow Flycatcher survey and historical breeding areas at Marine Corps Base Camp Pendleton, 2021��4 Map showing Southwestern Willow Flycatcher conspecific playback survey plots, control plots, automated playback unit locations, and historical breeding territories at Marine Corps Base Camp Pendleton, 2021��6 Graph showing number of resident Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton, 2000–21��11 Map showing Willow Flycatcher occupancy in conspecific playback and control survey plots at Santa Margarita River (upstream) and Lake O’Neill in Marine Corps Base Camp Pendleton, 2021��12 Map showing Willow Flycatcher occupancy in conspecific playback and control survey plots at Santa Margarita River (downstream) and Pilgrim Creek in Marine Corps Base Camp Pendleton, 2021��13

Tables 1. 2. 3. 4.

Distribution of territorial Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton, 2000–21��10 Habitat characteristics of Willow Flycatcher locations at Marine Corps Base Camp Pendleton, 2021��14 Nesting activity of Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton, 2021��14 Logistic regression models for the effects of sex, year, and winter precipitation variables on annual survival of adult Southwestern Willow Flycatchers on Marine Corps Base Camp Pendleton, 2000–21��15 Parameter estimate, standard error, odds ratios, and 95-percent confidence intervals for the top two models explaining annual survival of adult Southwestern Willow Flycatchers on Marine Corps Base Camp Pendleton, 2000–21��16 Logistic regression models for the effects of age, year, and winter precipitation variables on annual survival of Southwestern Willow Flycatchers on Marine Corps Base Camp Pendleton, 2000–21��16 Parameter estimate, standard error, odds ratios, and 95-percent confidence intervals for the top model explaining annual survival of Southwestern Willow Flycatchers on Marine Corps Base Camp Pendleton, 2000–21��16

vii

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

Multiply

To obtain

Length centimeter (cm)

0.3937

inch (in.)

meter (m)

3.281

foot (ft)

kilometer (km)

0.6214

mile (mi)

Area hectare (ha)

2.471

acre

hectare (ha)

0.003861

square mile (mi2)

kilometer per hour (km/hr)

0.6214

mile per hour (mi/hr)

Datum Coordinate information is referenced to the World Geodetic System of 1984 (WGS 84) and the North American Datum of 1983 (NAD 83).

Abbreviations AICC

Akaike’s Information Criterion for small sample size

MCBCP

Marine Corps Base Camp Pendleton

PrecipEW

Precipitation total for early winter (October to December)

PrecipLW

Precipitation total for late winter (January to March)

Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 Annual Report By Scarlett L. Howell and Barbara E. Kus

Executive Summary The purpose of this report is to provide the Marine Corps with an annual summary of abundance, breeding activity, demography, and habitat use of the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton (MCBCP). Surveys for the flycatcher were completed at MCBCP between May 5 and July 31, 2021. All of MCBCP’s historically occupied riparian habitat (core survey area) was surveyed for flycatchers in 2021. Additionally, one-fifth of the non-historically occupied riparian habitat (non-core survey area C) was surveyed for flycatchers. Twenty-four transient Willow Flycatchers of unknown subspecies were observed on five of the seven drainages surveyed in 2021. No Willow Flycatchers were detected at French or Las Flores Creeks. Transients occurred in a range of habitat types, including mixed willow (Salix spp.) riparian, riparian scrub, willow-sycamore (Platanus sp.) dominated or oak (Quercus spp.) sycamore-dominated riparian, and non-native-dominated riparian habitat. Exotic vegetation, primarily poison hemlock (Conium maculatum), was present in most flycatcher locations. The resident population of Southwestern Willow Flycatchers on MCBCP declined 50 percent, from two individuals in 2020 to one individual in 2021. In 2021, the resident Southwestern Willow Flycatcher population on MCBCP consisted of one unpaired female occupying one territory. No males were observed in 2021. The resident flycatcher population was restricted to the Santa Margarita River, and distribution was limited to the Pueblitos breeding area. The resident flycatcher territory was located in mixed willow riparian habitat. Nesting was initiated in late May and continued into late July. Two nesting attempts were documented, neither of which were successful. Infertile eggs likely accounted for both nest failures. No instances of Brown-headed

Cowbird (Molothrus ater) parasitism were observed. The female flycatcher placed her nests in native sandbar willow (Salix exigua) and used the same nest location for both nesting attempts. Of resident birds that were present at MCBCP in 2021, 100 percent were banded in previous years; no unbanded birds were detected. Of the two uniquely banded adult flycatchers (one male, one female) present during the 2020 breeding season, 50 percent (one female) returned to MCBCP in 2021. The banded female returned to the same breeding area and territory she occupied in 2020. Neither of the two nestlings banded in 2020 returned to MCBCP in 2021, and neither were detected off Base. From 2000 to 2021, adult over-winter survival of Southwestern Willow Flycatchers on MCBCP was 60±3 percent (mean±standard error [SE]), and first-year survival was 20±3 percent. A conspecific attraction study that used automatic playback units to broadcast flycatcher vocalizations in order to encourage flycatchers to settle on MCBCP was initiated in 2018 and repeated annually through 2021. The single resident flycatcher (female) detected in 2021 settled close to an automated playback unit.

Introduction The purpose of this report is to provide the Marine Corps with an annual summary of abundance, breeding activity, demography, and habitat use of the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton (MCBCP). The results are intended to provide the Base with biological information during each year to assist with appropriate management of the flycatcher and maintain compliant actions supporting military training on MCBCP in accordance with the Base Integrated Natural Resources Management Plan.

2 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher The Southwestern Willow Flycatcher is one of four subspecies of Willow Flycatcher in the United States, with a breeding range including southern California, Arizona, New Mexico, extreme southern parts of Nevada and Utah, southwestern Colorado, and western Texas (Hubbard, 1987; Unitt, 1987; Browning, 1993). Restricted to riparian habitat for breeding, the Southwestern Willow Flycatcher has declined in recent decades primarily in response to widespread habitat loss throughout its range and, possibly, Brown-headed Cowbird (Molothrus ater; hereafter “cowbird”) parasitism (Wheelock, 1912; Willett, 1912, 1933; Grinnell and Miller, 1944; Remsen, 1978; Garrett and Dunn, 1981; Unitt 1984, 1987; Gaines, 1988; Schlorff, 1990; Whitfield and Sogge, 1999). 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 Willow Flycatcher was listed as endangered by the State of California in 1990 (all subspecies; State of California, 2024) and by the U.S. Fish and Wildlife Service in 1995 (extimus subspecies, U.S. Fish and Wildlife Service, 1995). Southwestern Willow Flycatchers in southern California co-occur with the Least Bell’s Vireo (Vireo bellii pusillus; hereafter “vireo”), another riparian obligate endangered by habitat loss and cowbird parasitism. However, unlike the vireo, which has increased ten-fold since the mid-1980s in response to management alleviating these threats (U.S. Fish and Wildlife Service, 2006), Willow Flycatcher numbers have remained low. Currently, most Southwestern Willow Flycatchers in southern California are concentrated at two sites: (1) the Owens River valley in Inyo County (Greene and others, 2021) and (2) the upper San Luis Rey River at Lake Henshaw in San Diego County (Howell and Kus, 2021). Outside of these sites, Southwestern Willow Flycatchers occur as small, isolated populations of one to a half-dozen pairs. Not all sites are occupied annually; several sites in San Diego County have been occupied by small populations intermittently, including Whelan Lake, Guajome, Bonsall, and Couser Canyon on the San Luis Rey River, San Dieguito River, San Diego River, and Sweetwater River (Unitt, 1987; Kus and others, 2003). Male Southwestern Willow Flycatchers typically begin arriving in southern California early to mid-May, whereas females arrive approximately 1 week later (Sogge and others, 2010). Males sing repeatedly from exposed perches while on the breeding grounds. Once the pair bond is established, the

female builds an open-cup nest, usually placed in a branch fork of a willow (Salix spp.) or plant with a similar branching structure, approximately 1–3 meters (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/early September to their wintering grounds in southern Mexico, Central America, and northern South America (Sogge and others, 2010). The population of Southwestern Willow Flycatchers on MCBCP was at one time one of the largest in southern California (Unitt, 1987). Flycatcher numbers peaked in 2004 at 42 individuals, followed by declines that began in 2005. In response to declining flycatcher numbers on Base, a conspecific attraction study was initiated in 2018 to explore if broadcasting flycatcher vocalizations (conspecific playback) could be used to attract breeding flycatchers to recolonize MCBCP. Conspecific attraction, the tendency for individuals of a species to settle near one another, has been successfully used as a tool for recolonizing restored Sierra Nevada meadows by Willow Flycatchers of the adastus and brewsteri subspecies (Schofield and others, 2018), but has not been applied to any populations of the extimus subspecies. This report presents preliminary evaluation of Southwestern Willow Flycatcher response to conspecific playback (see the “Conspecific Playback Survey Methods” section). The purpose of this study, which began in 2000, was to document the status of Southwestern Willow Flycatchers on MCBCP in San Diego County, California. Specifically, our goals were to (1) determine the size and composition of the Willow Flycatcher population on Base, (2) document survival and movement of resident flycatchers, (3) document nesting activities, (4) characterize habitat used by flycatchers, and (5) evaluate the use of conspecific playback to attract breeding Southwestern Willow Flycatchers to historically occupied habitat to facilitate recolonization. These data, combined with data from 2000 to 2020, will inform natural resource managers about the status of this endangered species at MCBCP and guide modification of land use and management practices as appropriate to facilitate the species’ continued existence. This work was performed in cooperation with the Assistant Chief of Staff (AC/S), Environmental Security, Resources Management Division, Marine Corps Base Camp Pendleton, California. All activities were authorized by federal 10(a)1(A) Recovery Permit ESPER0004080_0.

Study Areas and Methods 3

Study Areas and Methods

4. Las Flores Creek, Lower Las Flores North (app. 1, fig. 1.3). 5. San Mateo Creek, Upper San Mateo (app. 1, fig. 1.5).

Population Size and Distribution In 2019, a reduced monitoring plan was implemented in which annual surveys for flycatchers occurred only in “core” survey areas, where breeding had historically been documented on Base. The remaining unoccupied riparian habitat was divided into five “non-core” survey groups, with each group surveyed on a rotational schedule every 5 years, beginning in 2020. In 2021, all of MCBCP’s historically occupied riparian habitat (core survey area; fig. 1) was surveyed for Willow Flycatchers. Additionally, one-fifth of the non-historically occupied riparian habitat (non-core C; fig. 1) was surveyed for flycatchers. Protocol surveys were completed four times between May 14 and July 31 (fig. 1; app. 1, figs. 1.1–1.6). Field work was completed by U.S. Geological Survey (USGS) personnel: Lisa Allen, Armand Amico, Annabelle Bernabe, Aaron Gallagher, Alexandra Houston, Scarlett Howell, Suellen Lynn, Rachelle McLaughlin, Shannon Mendia, Devin Taylor, and Michelle Treadwell. The specific areas surveyed were as follows:

Core Areas 1. Santa Margarita River: a. Above Hospital, Below Hospital West (app. 1, fig.1.1). b. Air Station East, Effluent Seep, Bell North, Bell South, Pump Road North, Rifle Range (app. 1, fig. 1.2). 2. Fallbrook Creek, Lake O’Neill (app. 1, fig. 1.1). 3. Las Flores Creek, Upper Las Flores North (app. 1, fig. 1.3). 4. San Mateo Creek, Lower San Mateo Bottom (app. 1, fig. 1.4). 5. Pilgrim Creek, South of Vandegrift (app. 1, fig. 1.6).

Rotating Non-Core Areas: C 1. Santa Margarita River, Pump Road South, Stagecoach (app. 1, fig. 1.2). 2. De Luz Creek, South (app. 1, fig. 1.1). 3. French Creek (fig. 1; app. 1, fig. 1.2).

Investigators followed standard survey protocol (Sogge and others, 2010), moving slowly (approximately 2 kilometers per hour [km/hr]) through the riparian habitat, while searching and listening for Willow Flycatchers. Observers walked along the edge(s) of the riparian corridor on the upland or river side, where habitat was narrow enough to detect a bird on the opposite edge. In wider stands, observers traversed the habitat, choosing routes that permitted detection of all birds throughout its extent. Surveys typically began at sunrise and were completed by early afternoon, avoiding conditions of high winds and extreme heat that can reduce bird activity and detectability. Upon initiation of the survey, investigators stood quietly for 1–2 minutes (min), listening for spontaneously singing Willow Flycatchers and acclimating to surrounding conditions, such as road noise, air traffic, and other bird songs. If no flycatchers were detected during the initial listening period, investigators broadcasted the Willow Flycatcher song (fitz-bew), using an MP3 player or phone and an amplified speaker, at the volume of typical bird songs for approximately 10–15 seconds (s), then looked and listened for approximately 1 min for a response. Song playback was ceased immediately upon detection of a Willow Flycatcher. Willow Flycatchers typically responded by moving silently toward the song, singing in response to the song, or responding with some other call or vocalization. This procedure was repeated (including a 10-s, quiet, pre-broadcast listening period) every 20–30 m throughout the survey site, more often if background noise was loud. If a Willow Flycatcher was detected, the investigator moved approximately 80–100 m beyond the detection before implementing additional playback to avoid double counting birds. Because Empidonax flycatchers look very similar and species other than Willow Flycatchers may be present in the habitat, identification of Willow Flycatchers was not made by sight alone; the primary fitz-bew song was required for detection purposes (Sogge and others, 2010). If a potential Willow Flycatcher responded silently, approached, or responded with another vocalization (for example, whitts) but did not sing, observers carefully backed away and waited quietly. In most cases, if the bird was a Willow Flycatcher, it sang within a short time (5–10 minutes). Flycatchers that did not sing by the end of the encounter but were suspected to be breeding were re-visited within 3 days (see the “Breeding Productivity Methods” section). Flycatchers that did not sing and were not suspected to be breeding individuals were not counted in survey results, unless the flycatcher was detected again in a subsequent survey period (Sogge and others, 2010).

4 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 117.58333

117.50000

117.41666

117.33333

117.25000

Redding

33.50000

Sacramento

San Mateo Creek

San Francisco

CALIFORNIA Cristianitos Creek

Los Angeles

Study area San Diego

Piedra de Lumbre Canyon Santa Margarita River

33.41666 San Mateo

Roblar Creek De Luz Creek Above Hospital

Las Flores Creek

San Onofre Creek

Las Flores

33.33333

Below Hospital

Horno Canyon

Air Station

Rifle Range

Treatment Ponds

Pump Road

Pueblitos

Fallbrook Creek Lake O'Neill

Pilgrim Creek Pilgrim

Ysidora Ponds Bell

Aliso Creek

Vine

French Creek

33.25000

EXPLANATION Historical breeding area Survey area Core Non-core A Non-core B Non-core C Non-core D Non-core E Base map from Esri and its licensors, copyright 2023 Universal Transverse Mercator, zone 11 north North American Datum of 1983

Windmill Canyon

Cockleburr Canyon Newton Canyon Santa Margarita River

0 0

2.5 2.5

5 MILES 5 KILOMETERS

Figure 1. Southwestern Willow Flycatcher survey and historical breeding areas at Marine Corps Base Camp Pendleton, 2021. White labels describe drainages; blue labels describe historical breeding areas (Core and Non-core C surveyed in 2021).

Study Areas and Methods 5 For each bird encountered, investigators recorded age (adult or juvenile), breeding status (paired, undetermined, or transient), and if possible, whether the bird was banded. Flycatcher locations were mapped using ESRI Collector release 20.2.2 (Environmental Systems Research Institute, 2020) 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 Geodetic System 1984 [WGS 84]). For all resident flycatchers, territory boundaries were approximated by mapping singing perches and the extent of the male’s or female’s use area. Resident flycatchers were named according to the historical territory where they established a territory in 2021 (for example, ETC). Transient flycatchers were named using a combination of numbers and letters according to the survey location where they were detected (for example, PR equals Pump Road), and numbered in the order in which they were detected, followed by the letter F to represent that the bird was a flycatcher.

Habitat Characteristics Habitat was characterized by visual inspection at each flycatcher location. For all resident flycatchers, habitat type was assessed at the territory level. For paired birds, habitat type was assessed within the male’s territory boundary, except for those pairs that included polygynous males, in which case habitat type was assessed within the females’ use areas. Habitat type was recorded according to the categories below, based on dominant vegetation: Mixed willow riparian: Habitat dominated by one or more willow species, including Goodding’s black willow (Salix gooddingii), arroyo willow (Salix lasiolepis), and red willow (S. laevigata), with mule-fat (Baccharis salicifolia) as a frequent co-dominant. Willow-cottonwood: Willow riparian habitat in which Fremont’s cottonwood (Populus fremontii) 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 coast live oak (Quercus agrifolia) occur as co-dominants.

Riparian scrub: Dry or sandy habitat dominated by sandbar willow (S. exigua) or mule-fat, with few other woody species. Upland scrub: Coastal sage scrub adjacent to riparian habitat. Non-native: Areas vegetated primarily with non-native species, such as giant reed (Arundo donax) and saltcedar (Tamarix ramosissima). Percent cover of exotic vegetation at each location was estimated using cover categories of less than 5 percent, 5–50 percent, 51–95 percent, and greater than 95 percent, and the dominant exotic species was recorded.

Conspecific Playback Survey Methods Historical breeding territories at MCBCP were grouped into 14 plots; 7 of the plots received conspecific vocalization broadcasts designed to attract Southwestern Willow Flycatchers, and the remaining 7 served as a control group, receiving no vocalization broadcasts (fig. 2; Schofield and others, 2018). Of the seven conspecific playback plots, five were located on the Santa Margarita River, one was at Lake O’Neill, and one was at Pilgrim Creek. Of the control plots, six were located on the Santa Margarita River, and the remaining plot was located on Pilgrim Creek. In the conspecific playback plots, an automated unit broadcasted a combination of Willow Flycatcher vocalizations (primary fitz-bew) song and various calls, interspersed with silence) from 0100 to 0600, 0700 to 0900, and 2000 to 2100 Pacific Standard Time (PST). Vocalizations were broadcast at a volume level mimicking the typical level of spontaneously singing Willow Flycatchers and could be heard by observers at a maximum of 80 m away from the broadcast unit. Automated broadcast units consisted of a FOXPRO NX4 wildlife caller (FOXPRO, Lewiston, Pennsylvania, USA) connected to a Favolcano CN101A digital programmable timer (Favolcano, Fujian, China) and an external power source (12-volt, 9 amp-hours, AH, battery). The automated broadcast units were operated from April 28 to August 30, 2021, spanning the time when northbound flycatchers would be searching for locations to settle and potentially encompassing southbound flycatchers that might settle in future breeding seasons. Control plots not receiving broadcast vocalizations were located a minimum of 200 m from the broadcast unit to eliminate influence from the broadcast.

6 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 117.38333

117.36667

Santa Margarita River

Artificial seep

HP3

HP4 Artificial seep HN3

33.28333

HP5

33.33889

117.32778

117.32222

Lake O'Neill HP2

HN4 33.33333

HN5

HN2

33.32778

HN6 33.26667

117.30556 32.28333

HP6

117.30000

Pilgrim Creek HP1 HP1

EXPLANATION Survey plot and identifier

HN7

HP6 Conspecific playback 33.27778

HN7 Control

Automated playback unit Historical breeding territory

33.25000

0 0

0.25 0.25

0.5 MILE

HP7

HN1

33.27222

0.5 KILOMETER

Figure 2. Southwestern Willow Flycatcher conspecific playback survey plots, control plots, automated playback unit locations, and historical breeding territories at Marine Corps Base Camp Pendleton, 2021.

Study Areas and Methods 7 Surveys were completed in the conspecific playback and control plots once per week in May and June and every other week in July to determine if any Southwestern Willow Flycatchers had established a territory in response to the conspecific playback. Surveys in 2021 began on May 5 and concluded on July 28. Investigators surveyed within 50 m of all historical breeding territories that were within the playback or control plot, following a slightly modified survey protocol that has been developed for attracting Willow Flycatchers to restored Sierra Nevada meadows (Schofield and others, 2018). Upon initiation of the survey, investigators stood quietly for 3–5 min, listening for spontaneously singing or calling Willow Flycatchers. If no flycatchers were detected during the initial listening period, investigators broadcasted the Willow Flycatcher song for approximately 30 s, then looked and listened for approximately 2 min for a response. If no response was detected, investigators repeated the 30-s broadcast and 2-min listening period. If no flycatchers were detected after the second round of broadcasting/listening, the investigator moved to the next historical location within the survey plot and repeated the sequence with a 1-min, pre-broadcast listening period before beginning the 30-s playback. In plots with automated broadcast units, the units were turned off before beginning the survey.

Artificial Seep Monitoring Three artificial seeps designed to increase the amount of surface water present in an effort to enhance Southwestern Willow Flycatcher breeding habitat were installed by MCBCP: one in 2019 and two in 2021. The seeps were located within three of the conspecific playback plots (fig. 2) along the Santa Margarita River. During conspecific playback surveys, we observed the habitat immediately surrounding the seeps and recorded if any Willow Flycatchers were using the area.

Breeding Productivity Methods Flycatchers observed during protocol surveys that were suspected to be resident birds (for example, observed in more than one survey period, pair vocalizations heard, evidence of nesting seen) were revisited within 3 days of the detection date. Resident birds were observed for evidence of nesting, and nests were located and monitored following standard protocol (Rourke and others, 1999). Nests were visited as infrequently as possible to minimize disturbance and reduce the chances of leading predators or cowbirds to nest sites. Typically, there were three to four visits per nest, spaced approximately 5–10 days apart, depending on the stage of the nest when initially detected. The first visit was timed to determine the number of eggs laid, the next to confirm

hatching and age of young, and the last to band nestlings. After a nest became inactive, six possible nest fates were assigned based on the parameters listed below: Successful (SUC): Nest fledged at least one young. Fledging was confirmed by detection of young outside the nest. Nest failed as a result of predation (PRE): This category included (1) nests seen in the process of ant or other predation; (2) nests found with evidence, such as eggshell fragments, feathers, or partially consumed nestlings in or below the nest; (3) nests with eggs or nestlings later found empty and torn from supporting branch, either partially or completely, typically indicative of mammal predation (Peterson and others, 2004); and (4) nests that had eggs or nestlings but were later found intact and empty before the expected fledge date with no evidence of eggs or nestlings on the ground, consistent with snake and bird predation, which typically leave no sign (Peterson and others, 2004). Nest failed as a result of parasitism (PAR): This category included (1) nests that were abandoned with one or more cowbird eggs in the nest and (2) nests that were tended by the host but contained only cowbird eggs. Incomplete (INC): Nests that were seen under construction but were never completed. Nest failed for other reasons that are known (OTH): This category included (1) nests that failed for reasons, such as host plant failure, surrounding vegetation falling and crushing a nest; (2) nests with inviable eggs that did not hatch after more than 2 weeks; and (3) nests that failed due to human disturbance, such as mowing or weed-whacking. This category also included nests that appeared to have failed as a result of cowbird “predation,” such as (1) abandoned nests containing punctured eggs in or below the nest, (2) nests where nestlings were killed by a puncture wound to the skull, or (3) nests where nestlings were ejected from the nest and found on the ground. Nest failed for unknown reasons (UNK): This designation was used when no other reason could be confirmed. In many instances, the fate “UNK” was assigned to nests that were likely depredated but, because we could not confirm egg-laying, did not fit the criteria of the “PRE” fate (above); these are explained more fully in the results.

8 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher

Nest Site Characteristics Nest site characteristics were recorded after the abandonment or fledging of nests. Measurements included nest height, host species, host height, distance from the nest to the edge of the host species, and distance from the nest to the edge of the clump of riparian vegetation (Rourke and others, 1999). Distance to edge of clump was expressed as a negative number if the nest was not located in a clump of riparian vegetation. For example, if the nest was in a field of poison hemlock (Conium maculatum) without any other non-hemlock, riparian vegetation present, the distance to the nearest clump of riparian vegetation was measured and the value expressed as a negative number.

Survivorship, Fidelity, and Movement To facilitate analyses of survival, fidelity, and movement, we attempted to capture and color band all resident flycatchers detected on MCBCP starting in 2000. Attempts were made each year to capture any unbanded adults within their territories using mist nets and band them with a numbered federal band on one leg and a solid or bi-colored metal band on the other. Returning adults previously banded as nestlings with a single numbered federal band were target netted to determine their identity, and their original band was supplemented with other bands to generate unique color combinations. Nestlings were banded at 7–10 days of age. Each bird received a silver, aluminum, federal numbered band on the right leg.

Annual Survivorship During surveys and nest monitoring activities, we attempted to resight all Willow Flycatchers to determine if they were banded, and if so, to confirm their identity by reading their unique color band combination or by recapturing birds with single federal bands. We used resighting and recapture data to calculate over-winter survival, or the probability of surviving from one year to the next. Annual survival was calculated for adults and first-year flycatchers that were banded as nestlings or juveniles (first-year survival). 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 are detected as banded but their full color combinations [and thus identities] are not obtained, or birds with single federal bands are not recaptured and thus their identities not determined). We analyzed annual survival of banded flycatchers from 2000 to 2021 at MCBCP with the Cormack-Jolly-Seber (CJS) model in Program MARK (White and Burnham, 1999) using the RMark package (Laake, 2013) in R (R Core Team, 2021). Survival analysis in Program MARK accounts for individuals that were present but not detected or captured by modeling both survival and recapture probability. Annual survival models were built by creating an encounter history matrix of all individual Southwestern Willow Flycatchers ever detected on MCBCP and if they were observed in each year from 2000 through 2021. Flycatchers were grouped by age (originally encountered as a hatch-year bird versus originally encountered as an adult) and sex (female versus male). To evaluate the effect of age, sex, year, and winter precipitation on detection probability, we built and compared models holding survival constant. For the adult-only models, we determined that detection probability was affected by sex, so sex was included in evaluating all models for adult flycatcher survival. For the model that included adults and first-year birds, we determined that detection probability was affected by age, so age was included in evaluating all models for our age-dependent flycatcher survival. This model set did not include sex because we were unable to determine sex of flycatchers banded as nestlings, unless they returned and were recaptured and identified as adults. Therefore, only the nestlings that survived their first winter could be classified retroactively as male or female, which would severely bias the estimate of sex-related survival of first-year flycatchers. Winter precipitation was grouped into two periods of each bio-year (July 1–June 30; Office of Water Resources, 2021): early winter (October–December of the calendar year preceding the breeding season [PrecipEW]) and late winter (January to March of the calendar year of the breeding season [PrecipLW]). We created a set of models using total winter precipitation (the sum of EW and LW precipitation [PrecipTW]), early winter precipitation, and late winter precipitation as independent variables, as well as an additive model which evaluated early and late winter precipitation together to estimate the coefficients for one variable while controlling for the other (PrecipEW + PrecipLW).

Results 9 We used an information-theoretic approach (Akaike’s Information Criterion for small sample size, or AICc) to evaluate support for models that tested the effects of age, sex, year, and winter precipitation on survival (Burnham and Anderson, 2002). We used logistic regression with a logit link to build and rank models by AICc, and identified the top model. First, we generated a constant survival model to serve as a reference for the effects of age, sex, year, and winter precipitation on survival. We then modeled the covariates and evaluated support for the models in relation to the constant survival model. We considered models “well-supported” if they were within 2 AICc units of the top model (ΔAICc) and had an Akaike Information Criterion (AIC) weight (the weight of evidence for the given model relative to the other models in the set) greater than 0.05. We present real estimates of annual survival from the top model. If there were multiple well-supported models (ΔAICc less than or equal to 2), we averaged over them using AIC weights to obtain annual real estimates of survival for adult females, adult males, all adults, and all first-year flycatchers. We also evaluated the effects of covariates within our top models by calculating the odds ratio for each covariate (the odds that the covariate had an effect on survival, where “no effect” is equal to 1, negative effect is less than 1, and 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 span 1, we concluded that we had 95-percent confidence that the covariate had a positive or negative effect on survival.

Site Fidelity and Movement Site fidelity and between-year and within-year movements of flycatchers were determined by measuring the distance between the center of a flycatcher’s breeding or natal territory in 2020 and the center of the same flycatcher’s breeding territory in 2021. Flycatchers demonstrated site fidelity if they returned within 100 m of their 2020 territory.

Data Comparisons Data from most years at MCBCP used in comparisons with current data can be found in the following: Kus, 2001; Kus and Ferree, 2003; Kus and Kenwood, 2003, 2005, 2006a,

2006b; Kenwood and Kus, 2007; Rourke and others, 2008; Howell and Kus, 2009a, 2009b, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2024.

Results Population Size and Distribution Transients Twenty-four Willow Flycatchers of unknown subspecies were observed during protocol and conspecific playback surveys (app. 2, figs. 2.1–2.5). All transients were detected between May 19 and June 25. Transients were detected on five of the seven drainages surveyed in 2021. No Willow Flycatchers were detected on French or Las Flores Creeks.

Residents One Southwestern Willow Flycatcher (female) was detected during the 2021 breeding season (table 1; app. 2, fig. 2.2; app. 3, fig. 3.1). The resident female was initially detected during conspecific playback surveys. One territory was established, consisting of one unpaired female. No single males or non-territorial floaters were observed in 2021. Overall, the resident flycatcher population on MCBCP decreased by 50 percent (two individuals to one individual) from 2020 to 2021 (fig. 3). Resident flycatchers were restricted to the Santa Margarita River (app. 2, fig. 2.2; app. 3, fig. 3.1). In 2021, only the Pueblitos flycatcher breeding area along the Santa Margarita River was occupied, which hosted the unpaired female (table 1). Overall, flycatcher distribution on the Santa Margarita River remained contracted relative to previous years. Multiple breeding areas along the Santa Margarita River that historically supported resident flycatchers (Vine, Bell, Ysidora Ponds, Pump Road, Treatment Ponds, and Air Station) were devoid of flycatcher territories in 2021 (fig. 1; table 1).

[Drainage/breeding area: FC, Fallbrook Creek; LF, Las Flores Creek; PC, Pilgrim Creek; SO, San Mateo Creek; SR, Santa Margarita River]

Drainage/ breeding area

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

2021

FC/Lake O'Neill

LF/Las Flores

PC/Pilgrim

SO/San Mateo

SR/Above Hospital

SR/Below Hospital

SR/Air Station

SR/Rifle Range

SR/Pump Road

SR/Treatment Ponds

SR/Pueblitos

SR/Ysidora Ponds

SR/Bell

SR/Vine

SR/Stuart Mesa

Total

1One male's territory overlapped two breeding areas, included in Treatment Ponds total. 2Two different females utilized additional breeding areas (Air Station and Treatment Ponds) before settling in Pueblitos, included in Pueblitos total.

10 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher

Table 1. Distribution of territorial Southwestern Willow Flycatchers (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton, 2000–21.

Results 11 45

EXPLANATION

Male Female Unknown sex

Number of resident flycatchers

35 30 25 20 15 10 5

2019

2021

2017

2019

2015

2017

2013

2015

2011

2013

2009

2011

2007

2009

2005

2007

2003

2005

2001

2003

0 2001

2021

Year

Figure 3. Number of resident Southwestern Willow Flycatchers (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton, 2000–21.

Conspecific Playback Surveys

Breeding Productivity

We detected resident flycatchers in 14 percent (1/7) of conspecific playback plots and none of the control plots (figs. 4, 5). The resident flycatcher (female) detected on Base in 2021 settled within 165 m of an automated playback unit. Transient flycatchers were detected in 43 percent (3/7) of conspecific playback plots and 14 percent (1/7) of control plots with no playback.

Nesting was observed for one unpaired female and was initiated in late May (table 3). The earliest estimated egg-laying date was May 22, and the latest was June 21. Nesting continued into late July. During the 2021 breeding season, two nesting attempts were documented, neither of which successfully fledged flycatcher young. Both nests ultimately failed, most likely as a result of infertile eggs. After the eggs in nest one did not hatch within 25+ days, the female added new nest material, buried the unhatched clutch under nest two, and laid a new clutch. Mean clutch size (±SD) calculated from two nests known to have full clutches was 3.5±0.7 eggs.

Habitat Characteristics In 2021, 64 percent (16/25) of flycatcher locations occurred in habitat classified as mixed willow riparian, 81 percent (13/16) of which occurred along the Santa Margarita River (table 2). Another 16 percent (4/25) of locations were in riparian scrub, dominated by mule-fat or sandbar willow, and 12 percent (3/25) of locations were in willow habitat co-dominated by sycamore. The remaining two detections were located either in more arid habitat, dominated by a mix of oak and sycamore, or in habitat consisting of primarily non-native vegetation. Exotic vegetation occurred in most flycatcher locations in 2021 (15/25). The most common exotic plant in habitat used by flycatchers was poison hemlock (table 2). Of all flycatcher locations, 28 percent (7/25) were composed of 5–50 percent exotic vegetation, and 24 percent (6/25) were dominated by exotic vegetation (percent cover of exotics greater than 50 percent).

Nest Site Characteristics The flycatcher nesting at MCBCP in 2021 placed nests in only one species of plant: sandbar willow. Nest height was 1.4 m, whereas host height was 2.7 m. The nest was placed 0.3 m from the edge of the willow and 2.0 m from the edge of the clump of vegetation.

Cowbird Parasitism All nests were checked for the presence of cowbird eggs. No nest parasitism of Southwestern Willow Flycatcher nests by cowbirds was documented in 2021.

12 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 117.37778

117.37278

117.36667

Santa Margarita River 33.29444

Lake O’Neill inset

Study area

Air Station

Map area

HP3 Santa Margarita River inset 117.32500 32.34166

Lake O'Neill

117.32083

Lake O'Neill

33.28889

Rifle Range

HP4

Lake O'Neill 33.33750

HP2 Treatment Ponds 33.28333

HN3 HP5

33.33333

Pump Road

HN4

33.27778

EXPLANATION Survey plot and identifier

HP5 Conspecific playback

Pueblitos

HN5 Control 2021 Willow Flycatcher locations Historical breeding areas

HN5

Transient Historical breeding territory Resident

0 0

0.1 0.1

0.2 MILE 0.2 KILOMETER

Figure 4. Willow Flycatcher occupancy in conspecific playback and control survey plots at Marine Corps Base Camp Pendleton, 2021: Santa Margarita River (upstream) and Lake O’Neill.

Results 13 117.37778

117.37222

117.36667

HN5

Santa Margarita River

EXPLANATION Survey plot and identifier

Pueblitos

HP7 Conspecific playback HN7 Control 2021 Willow Flycatcher locations

HN2

Historical breeding areas Transient

33.26667

Historical breeding territory

HN6 117.30556

Ysidora Ponds

117.30278

Pilgrim Creek 33.28056

HP1

HP6

33.26111

33.27778

Pilgrim

Bell

33.27500

HN7

HN1

33.25556

Study area

Vine

HP7

Map area Santa Margarita River inset

33.25000

0.1

Pilgrim Creek inset

0.2 MILE

0.2 KILOMETER

Figure 5. Willow Flycatcher occupancy in conspecific playback and control survey plots at Marine Corps Base Camp Pendleton, 2021: Santa Margarita River (downstream) and Pilgrim Creek.

14 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher Table 2. Habitat characteristics of Willow Flycatcher (Empidonax traillii) locations at Marine Corps Base Camp Pendleton, 2021. [Bird identifier: name that represents a resident flycatcher territory or a transient location. Breeding status: S, resident unpaired female; T, transient. Exotic cover class: 1, less than 5 percent; 2, 5–50 percent; 3, 51–95 percent. Dominant exotics: BRA, black mustard (Brassica nigra); CON, poison hemlock; EUC, eucalyptus (Eucalyptus sp.); PIC, ox-tongue (Helminthotheca echioides). Other abbreviation: —, no data]

Bird identifier

Drainage

Breeding status

Habitat type

Exotic cover class

Dominant exotics

DS01F

De Luz Creek

Willow/sycamore

—

HP201F

Fallbrook Creek

Mixed willow

—

OL01F

Fallbrook Creek

Mixed willow

—

PS01F

Pilgrim Creek

Mixed willow

—

PS02F

Pilgrim Creek

Oak/sycamore

—

MB01F

San Mateo Creek

Riparian scrub

BRA, CON

MB02F

San Mateo Creek

Riparian scrub

BRA, CON

MU01F

San Mateo Creek

Willow/sycamore

—

MU02F

San Mateo Creek

Willow/sycamore

—

ES01F

Santa Margarita River

Mixed willow

—

ETC

Santa Margarita River

Mixed willow

CON

HN701F

Santa Margarita River

Mixed willow

CON

HP501F

Santa Margarita River

Mixed willow

CON

HP601F

Santa Margarita River

Mixed willow

CON

PR01F

Santa Margarita River

Mixed willow

—

PR02F

Santa Margarita River

Mixed willow

—

PR03F

Santa Margarita River

Mixed willow

BRA

RR01F

Santa Margarita River

Riparian scrub

CON

RR02F

Santa Margarita River

Non-native

EUC

RR03F

Santa Margarita River

Mixed willow

CON

SG01F

Santa Margarita River

Mixed willow

PIC

SG02F

Santa Margarita River

Mixed willow

BRA

SG03F

Santa Margarita River

Riparian scrub

BRA

SG04F

Santa Margarita River

Mixed willow

CON

SG05F

Santa Margarita River

Mixed willow

CON

Table 3. Nesting activity of Southwestern Willow Flycatchers (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton, 2021. [Nest fate: OTH, Nest failed for other reasons that are known. Other abbreviations: ID, identifier; mm-dd-yy, month-day-year; #, number; +, plus]

Bird ID

Lay date # # # (mm-dd-yy) Eggs Nestlings Fledglings

Nest fate

ETC

05/22/2021

OTH Nest found with four eggs, completion and lay date estimated. Eggs infertile, did not hatch within 25+ days. Four eggs still present on 6/15, next visit eggs buried and nest two had been completed on top of nest one.

06/21/2021

OTH Eggs infertile, did not hatch within 19+ days. Nest tilted between 6/30 and 7/12. Nest inactive with one cold egg on 7/19.

Comments

Results 15

Survivorship, Site Fidelity, and Movement Overview of Banded Population All Southwestern Willow Flycatchers were observed to determine with confidence if they were banded. The resident female was banded as an adult in 2013 at MCBCP, making her at least 9 years old. No new adults or nestlings were banded in 2021.

Annual Survivorship The recapture and resighting of banded birds allowed us to determine the proportion of Southwestern Willow Flycatchers previously documented on MCBCP that returned to hold territories in 2021. Of the two uniquely banded adult flycatchers (one male, one female) present during the 2020 breeding season, 50 percent (1/2) returned to MCBCP in 2021. Neither of the two nestlings banded in 2020 that survived to fledge were resighted or recaptured at MCBCP in 2021.

Survivorship Models—Adults Only Of the eight models we built, the two highest ranked models were well supported, with AICc weights greater than 0.20 and the difference in AICc between each model and the best model (ΔAICc) within 0–2 (table 4). Sex appeared in the top model, and the difference between female and male survival was significant (95-percent confidence interval

of the odds ratio did not include 1; table 5). Based on the average of models with ΔAICc less than 2, female annual survival was 67±4 percent (mean ± standard error [SE]), whereas male annual survival was 53±5 percent. Detection probability for males (94±4 percent) was higher than for females (75±6 percent). Total winter precipitation (PrecipTW) appeared in the second highest ranked model; however, the 95-percent confidence interval of the odds ratios included 1, and the odds ratio equaled 1, indicating that this variable had little effect on survival. Models including winter precipitation in varying time intervals (PrecipLW, PrecipEW, Precip EW+Precip LW) had more support than the constant survival model, but the difference in AICc between these models and the best model (ΔAICc) was greater than 2. Year appeared only in the two lowest ranked models and was not as well supported as the constant survival model.

Survivorship Models—Adults Versus First-Year Flycatchers Of the eight models built to examine the effects of age, year, and winter precipitation on annual survival of all flycatchers, the top model included age (table 6). This model had an AICc weight greater than 0.99, well above any other model in the model set. In this top model, survival differences between first-year and adult flycatchers were significant (95-percent confidence interval of the odds ratio did not include 1; table 7). Based on the top model that included age, adult survival was 60±3 percent (mean±SE), whereas first-year survival was 20±3 percent.

Table 4. Logistic regression models for the effects of sex (male versus female), year, and winter precipitation variables on annual survival of adult Southwestern Willow Flycatchers (Empidonax traillii extimus) on Marine Corps Base Camp Pendleton, 2000–21. [The effect of sex on detection probability was included in all models. Models are ranked from best to worst based on Akaike’s Information Criteria for small samples (AICc), change in AICc (ΔAICc), and AICc weights. AICc is based on −2×loge likelihood and the number of parameters in the model. Abbreviation: +, plus]

AICc

ΔAICc

AICc weight

Deviance

Number of parameters

Sex

50008.1

0.0

0.37

49773.3

PrecipTW

50009.1

0.9

0.23

49774.3

PrecipLW

50010.3

2.2

0.12

49775.5

PrecipEW

50010.4

2.3

0.12

49775.6

PrecipEW+PrecipLW

50011.1

2.9

0.08

49774.2

Model

Constant

50011.2

3.1

0.08

49778.5

Sex+Year

50023.4

15.3

0.00

49744.9

Year

50027.6

19.4

0.00

49751.4

16 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher Table 5. Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for the top two models explaining annual survival of adult Southwestern Willow Flycatchers (Empidonax traillii extimus) on Marine Corps Base Camp Pendleton, 2000–21. [Models are in order of best-supported to least-supported. Model: PrecipTW, total winter precipitation from October 1 of the calendar year before the breeding season to March 31 of the breeding season year]

Model

Odds ratio

Intercept1

0.7

0.2

2.0

1.41–2.92

Male

−0.6

0.3

0.6

20.33–0.92

Intercept

−0.1

0.3

0.9

0.49–1.63

PrecipTW

0.0

1.0

31.00–1.04

Effect

Sex PrecipTW

95-percent CI

1Intercept includes females. 2The 95-percent confidence interval of the odds ratio does not span 1. 3Value before rounding up was below 1.00.

Table 6. Logistic regression models for the effects of age (first-year versus adult), year, and winter precipitation variables on annual survival of Southwestern Willow Flycatchers (Empidonax traillii extimus) on Marine Corps Base Camp Pendleton, 2000–21. [The effect of age on detection probability was included in all models. Models are ranked from best to worst based on Akaike’s Information Criteria for small samples (AICc), change in AICc (ΔAICc), and AICc weights. AICc is based on −2×loge likelihood and the number of parameters in the model. Models: PrecipEW, early winter precipitation from October 1 to December 31 of the calendar year before the breeding season; PrecipTW, total winter precipitation from October 1 of the calendar year before the breeding season to March 31 of the breeding season year; PrecipLW, late winter precipitation from January 1 to March 31 of the breeding season year. Abbreviation: +, plus]

Model

AICc

ΔAICc

AICc weight

Deviance

Number of parameters

Age

840.9

0.0

1.00

304.9

Age+Year

860.3

19.4

0.00

282.2

Constant

889.4

48.5

0.00

355.4

PrecipEW

889.5

48.6

0.00

353.5

PrecipTW

890.5

49.5

0.00

354.5

PrecipLW

891.2

50.3

0.00

355.2

PrecipEW+PrecipLW

891.6

50.6

0.00

353.5

Year

908.0

67.1

0.00

332.1

Table 7. Parameter estimate (β), standard error (SE), odds ratios, and 95-percent confidence intervals (CI) for the top model explaining annual survival of Southwestern Willow Flycatchers (Empidonax traillii extimus) on Marine Corps Base Camp Pendleton, 2000–21. [The Intercept includes first-year flycatchers. All other effects values are the difference between that parameter and the Intercept]

Model

Effect

Odds ratio

Age

Intercept

−1.4

0.2

0.17–0.35

6.0

13.84–9.37

Adult

1.8

0.2

95-percent CI

1The 95-percent confidence interval of the odds ratio does not span 1.

Discussion 17

Site Fidelity and Movement Southwestern Willow Flycatchers at MCBCP generally settle into historically occupied breeding areas to establish territories (see historical breeding areas; fig. 1). Resighting banded birds allowed us to examine between-year and within-year site fidelity. The one banded flycatcher (female) seen in both 2020 and 2021 returned to the same breeding area (Pueblitos) and territory last occupied. The female has occupied this area since 2018, after she moved mid-season from the Treatment Ponds area where she nested from 2013 to 2015. The female was not detected in 2016 or 2017. No instances of movement by adult Southwestern Willow Flycatchers were observed within the 2021 breeding season.

Discussion The resident population of Southwestern Willow Flycatchers on MCBCP in 2021 (one female) decreased 50 percent from 2020. There were no males detected on MCBCP during the 2021 breeding season. Although resident flycatchers continue to be present on Base, the current population has declined 98 percent from the record high of 42 individuals in 2004. An overall downward trend began in 2005, characterized by several stepwise population declines, where the resident population dropped more than 24 percent from 1 year to the next (2004–05: 24 percent; 2007–08: 42 percent; 2013–14: 47 percent; 2014–15: 44 percent; and 2015–16: 40 percent). No resident flycatchers were detected on Base in 2017, but in 2018, three resident flycatchers that were previously detected on MCBCP returned and resumed breeding. The resident population was stable at three individuals from 2018 to 2019, before resuming the downward trend with a 67-percent decline from 2019 to 2020. The number of transients detected annually in the core plus non-core C survey areas has varied greatly, despite consistent survey scope and effort, from a low of 3 in 2000 to a high of 56 in 2002. Factors influencing the migratory route and variable timing of transient Willow Flycatchers are not well understood, but MCBCP continues to provide habitat for Willow Flycatchers. Resident flycatchers were limited to the Santa Margarita River in 2021, whereas transient flycatchers were also located on De Luz, Fallbrook, Pilgrim, and San Mateo Creeks. Habitat use by resident flycatchers also differed from transient flycatchers; resident birds settled only in mixed willow riparian habitat, in contrast to transient flycatchers, which were

also found in riparian scrub, willow-sycamore, oak-sycamore, and non-native habitats. Exotic vegetation was also dominant in multiple transient locations, whereas the resident location had less than 50 percent exotic vegetation present. Of the seven primary breeding areas (those that supported multiple breeding pairs for at least 4 years) on the Santa Margarita River historically occupied by flycatchers, only the northern portion of Pueblitos was occupied in 2021. The northern portion of Pueblitos breeding area has been occupied by breeding birds most years since 2000, although since 2009, occupancy fluctuated between zero and one pair before reaching two pairs in 2018. In both 2019 and 2020, the northern portion of Pueblitos was occupied by a single pair, and was only occupied by an unpaired female in 2021. Between-year site fidelity in 2021 was 100 percent, with the female of the 2020 ETC pair returning to the same area and territory in 2021. Between-year site fidelity has been highly variable, ranging from a low of 40 percent in 2008 to a high of 100 percent in 2018, 2020, and 2021. Apart from 2016, site fidelity has been high since 2009 (75–100 percent), which may suggest that the occupied areas represent the most suitable habitat currently available on MCBCP. All other breeding areas more recently occupied by Southwestern Willow Flycatchers on MCBCP remained vacant in 2021, including the Treatment Ponds breeding area. The Treatment Ponds breeding area was last occupied by breeding birds in 2018; the area hosted one to four breeding pairs from 2006 to 2016, was unoccupied in 2017 (when no breeding birds were detected anywhere on MCBCP), and was occupied by one female at the beginning of the 2018 breeding season before she moved to Pueblitos. Two additional breeding areas were affected by fire in the recent past, which may have played a role in their occupancy status. The Air Station breeding area was last occupied by breeding flycatchers in 2019. The Air Station breeding area had been occupied by breeding birds on and off since 2000 and provided habitat for one to five breeding pairs in most years before the 2014 Las Pulgas fire, which burned approximately 350 hectares (ha) of riparian habitat along the Santa Margarita River (Howell and Kus, 2014). The Air Station breeding area has not been occupied by males since the fire; however, an unpaired breeding female attempted to nest in the area in 2016 and 2018, building a nest and laying infertile eggs both years. In 2018, the breeding female remained for approximately 1 month before moving to Pueblitos. In 2019, the breeding female returned to the Air Station breeding area and successfully fledged young. In 2020 and 2021, no breeding flycatchers were detected in the Air Station breeding area.

18 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher The Pump Road breeding area was last occupied in 2013, before the 2014 Las Pulgas wildfire (Howell and Kus, 2014), and the habitat may still be recovering. Based on habitat recovery after previous fires in Southwestern Willow Flycatcher-occupied habitat in other locations, a minimum of 3–5 years is necessary for burned habitat to become suitable again for breeding (Paxton and others, 2007). However, burned areas may take more than a decade to regain suitability for flycatchers after catastrophic wildfire and may not recover at all. A site along the San Pedro River in Arizona burned in June 1996 and still had not been reoccupied after 10 years (Durst and others, 2008), with the lack of suitable habitat regeneration suggested as the reason for extirpation (English and others, 2006). Habitat recovery after fire depends on many factors, including hydrologic conditions during regrowth; conditions such as drought, reduced groundwater, and altered river flow may impede recovery (Smith and others, 2009). Before the fire, occupancy at the Pump Road breeding area had been declining, from a high of 11 individuals in 2004 to 2 individuals the year before the fire. Possible reasons for the decline include habitat senescence, changes in vegetation composition and structure, or hydrologic changes. The three remaining primary breeding areas along the Santa Margarita River (Vine, Bell, and Ysidora Ponds) have been unoccupied since 2004, 2007, and 2008, respectively. The decline in these areas may have been related to prolonged overgrowth of exotic vegetation and poor recovery of the riparian habitat after removal. Exotic vegetation (giant reed and saltcedar) was removed from these areas in 2008 and 2009; however, the areas remain unoccupied. It is possible that drought conditions experienced in San Diego County slowed habitat recovery after exotic removal. In the past, flycatchers have reoccupied areas on MCBCP after exotic vegetation removal within 5–7 years: the Pump Road area had removal in 1996 and supported multiple breeding pairs by 2001; the Air Station area had removal in 2000 and had returned to pre-removal occupancy by 2007; and the Treatment Ponds area had removal in 2001 with recolonization by 2006 (Kus, 2001; Kenwood and Kus, 2007; Rourke and others, 2008). Mean (±SD) annual precipitation for the first 5 years after exotics removal for Pump Road, Air Station, and Treatment Ponds breeding areas was 41.9±23.9, 42.8±29, and 40.5±29.9 centimeters (cm), respectively (Office of Water Resources, 2021). In contrast, the mean annual precipitation for the first 5 years after exotics removal in 2008 (Ysidora Ponds) and 2009 (Bell and Vine) was 33.7±13.3 and 31.8±12.2 cm, respectively. Reduced precipitation in the post-removal years may have suppressed natural regrowth of riparian vegetation after removal. Although more precipitation occurred in more recent years (2017, 2019, and 2020), providing some relief from the drought, the 2018 and 2021 bio-years produced less than half the average

rainfall from 2000–20 (15.4 and 16.6 cm, respectively, versus 34.4±18.5 cm; Office of Water Resources, 2021), perpetuating drought conditions that do not favor vegetation growth. Although we suspect that fluctuations in annual precipitation, especially prolonged periods of drought, may have limited past habitat regrowth and the quantity of suitable breeding habitat in some areas, we did not see a clear relation between survival and precipitation when adult annual survival was modeled using RMark. Winter precipitation was included in one of the top two survival models as having a possible effect on survival; however, it was not a significant contributor to the model. The top model suggested that sex (male or female) was the primary driver of adult annual survival, with females having higher survival than males. Overall, first-year survival was lower than adult survival. Based on the model that included age, adult survival was 60±3 percent, whereas first-year survival was 20±3 percent. Because RMark factors in detection probabilities, first-year survival estimates were higher than zero for all years after 2012, despite the reality that no MCBCP fledglings have recruited into the on-Base breeding population since 2013. From 2001 to 2012, between one and six MCBCP natal birds annually would recruit into their natal site (MCBCP) and establish their first breeding territory. Since 2013, no juveniles that hatched at MCBCP have returned in subsequent years to breed anywhere on MCBCP. The reasons for this are unclear, but it is possible that, since 2013, first-year flycatchers have encountered unsuitable habitat conditions, such as habitat burned by the 2014 Las Pulgas fire. Any first-year flycatchers that attempted to recruit in 2014 would have encountered a large amount of burned habitat, which may have prompted them to settle elsewhere. Two MCBCP natal birds that fledged between 2013 and 2018 have been detected in other areas of San Diego County; a single male was discovered at the Otay River in 2014, 86 km from his 2013 natal site, and in 2015, a female was discovered breeding on the Upper San Luis Rey River near Lake Henshaw, 55 km from her 2013 natal site. First-year flycatchers have been documented dispersing as far as 444 km to establish their first breeding territory (Paxton and others, 2007). Because survival estimates cannot separate annual mortality from permanent emigration, especially in studies that only cover a small geographic area, it is possible that first-year survival is not low, and the birds simply dispersed elsewhere. The lack of recruitment to MCBCP for the past 9 years may be a result of low first-year survival, young birds dispersing to areas outside MCBCP, or both. While the small number of fledglings produced in the past 9 years may be contributing to the metapopulation in southern California as a whole, the failure of any recruits to return to MCBCP and perpetuate local breeding, for whatever reason, increases the likelihood of local extirpation in the near future.

Conclusions 19 Although it remains unclear what is driving the decline of resident flycatchers on MCBCP, several measures have been initiated in recent years in an attempt to reverse the declines, including using conspecific attraction to facilitate recolonization of historically occupied breeding areas. An experimental study using conspecific playback was initiated in 2018 and has continued annually through 2021. Conspecific attraction, the tendency for individuals of a species to settle near one another, has been successfully used as a tool for recolonizing restored Sierra Nevada meadows by Willow Flycatchers (Schofield and others, 2018). All resident flycatchers (one female) detected on MCBCP in 2021 settled within 165 m of automated playback stations; however, whether the female was attracted to the playback cannot be determined from the small sample size. The breeding area where the female settled in 2021 was recolonized in 2018 after a 1-year vacancy in 2017; none of the breeding areas with conspecific playback broadcasts that had been vacant for greater than 3 breeding years have been recolonized since the playback study was initiated. In 2021, site fidelity rather than conspecific attraction may have affected where the female settled, given that she nested within 25 m of her 2020 nest location. However, despite the uncertainty surrounding how conspecific attraction affects settlement patterns, it has the potential to play a role in attracting resident male flycatchers to MCBCP. In 2018, when the study was first initiated, a previously unknown male settled adjacent to the playback, allowing breeding activities to resume after no breeding flycatchers were detected on MCBCP in 2017. This male returned for two additional breeding seasons (2019, 2020) but was not present in 2021. Breeding in 2021 was unsuccessful because no male flycatcher was present. Additional conspecific attraction data could help determine if conspecific playback can be used to attract more flycatchers to MCBCP and potentially improve breeding success. The seep enhancement project initiated by AC/S Environmental Security to augment surface water in historically occupied habitat could help mitigate declines in the flycatcher population on MCBCP that could be related to potential changes in surface water. In 2021, no resident flycatchers were observed using the area adjacent to the seeps; however, the additional surface water provided by the seeps may benefit the riparian habitat. Increasing surface water may lessen the severity of drought effects in years with less than average rainfall, such as 2021 (16.6 cm versus average rainfall 2000–20, 34.4±18.5 cm; Office of Water Resources, 2021). The additional water may also assist in creating suitable

habitat conditions for breeding flycatchers. A combination of habitat restoration, seep enhancement, and subsequent conspecific broadcasts in restored and still suitable areas could help augment the Southwestern Willow Flycatcher population on MCBCP.

Conclusions The Southwestern Willow Flycatcher population in California appears to be experiencing a statewide decline not isolated to Marine Corps Base Camp Pendleton (MCBCP). Populations on the Kern River (Mary Whitfield, Southern Sierra Research Station, written commun., 2020), Bonsall on the San Luis Rey River (Allen and Kus, 2021), and the lower San Luis Rey River (Houston and others, 2021) have experienced steep declines or have been extirpated in recent years. Although resident flycatchers are still occupying MCBCP, the population has declined 98 percent from the high in 2004 to 2021. Habitat loss from fire, changes in habitat composition, and prolonged drought have likely contributed to the decline in resident Willow Flycatcher observations and could be detrimental to the long-term persistence of this endangered species on MCBCP. Therefore, carefully evaluating projects that have the potential to alter the habitat in currently occupied areas could help maintain suitable habitats in these areas. Based on our long-term observations of Southwestern Willow Flycatchers breeding under a variety of environmental conditions, the following actions could enhance habitat suitability and availability on MCBCP and thereby contribute to flycatcher recovery: 1. Evaluating potential changes in vegetation structure and composition that may have reduced the suitability of historically occupied areas by comparing vegetation data collected previously (in 2001 and 2002) with new data describing current habitat conditions (carried out in 2018 and 2019). 2. Implementing habitat restoration in previously burned (Pump Road) breeding areas. 3. Implementing habitat restoration in historically occupied breeding areas (Vine, Bell, and Ysidora Ponds) where exotics removal took place, but the vegetation has not yet regenerated, with priority given to areas closest to currently occupied habitat.

20 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 4. Creating or restoring water sources in or adjacent to historically occupied breeding areas and considering controlled release to mimic seasonal flooding, specifically at the following locations (World Geodetic System 1984 [WGS 84]): o Settling ponds (33.28351, −117.37373; 33.26235, −117.37162) o Canal/ditch (33.28254, −117.37361 [initiated in 2018]; 33.28053, −117.37371; 33.27817, −117.37476; 33.25122, −117.37616)

o Pooling (33.29325, −117.36784 [initiated in 2021]; 33.27908, −117.37424; 33.27874, −117.37338; 33.28443, −117.37991 [initiated in 2021]; 33.25797, −117.37241) With the continued decline of Southwestern Willow Flycatchers on MCBCP, communication between the AC/S Environmental Security and other military departments may become increasingly important. Our findings and experience indicate that impacts to active territories can be minimized when maintenance activities, such as clearing vegetation, are coordinated among personnel. This coordination and cooperation among various departments could help maintain a balance between the sometimes competing land uses on MCBCP, including military activities, recreation, habitat protection, and endangered species management.

References Cited 21

References Cited Allen, L.D., and Kus, B.E., 2021, Distribution and abundance of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Middle San Luis Rey River, San Diego County, southern California—2020 data summary: U.S. Geological Survey Data Series 1134, 11 p., accessed September 19, 2022, at https://doi.org/10.3133/ds1134. Browning, M.R., 1993, Comments on the taxonomy of Empidonax traillii (Willow Flycatcher): Western Birds, v. 24, no. 4, p. 241–257. Burnham, K.P., and Anderson, D.R., 2002, Model selection and multimodel inference—A practical information-theoretic approach (2d ed.): New York, N.Y., Springer-Verlag. Durst, S.L., Sogge, M.K., Stump, S.D., Walker, H.A., Kus, B.E., and Sferra, S.J., 2008, Southwestern Willow Flycatcher breeding sites and territory summary—2007: U.S. Geological Survey Open-File Report 2008–1303, 31 p., accessed on September 19, 2022, at https://pubs.usgs.gov/of/2008/1303/. English, H.C., Graber, A.E., Stump, S.D., Telle, H.E., and Ellis, L.A., 2006, Southwestern Willow Flycatcher 2005 survey and nest monitoring report: Phoenix, Ariz., Arizona Game and Fish Department, Nongame and Endangered Wildlife Program Technical Report 248. Environmental Systems Research Institute (Esri), 2020, Collector release 20.2.2: Redlands, Calif., Environmental Systems Research Institute. Gaines, D., 1988, Birds of Yosemite and the east slope: Lee Vining, Calif., Artemisia Press, 123 p. Garrett, K., and Dunn, J., 1981, Birds of southern California— Status and distribution: Los Angeles, Calif., Los Angeles Audubon Society, 408 p. Greene, L., Otto, E., and McCreedy, C., 2021, Owens Valley nesting willow flycatcher under pressure: California Fish and Wildlife Special CESA Issue, p. 286–297, accessed December 8, 2022, at www.doi.org/10.51492/cfwj.cesasi.17. Grinnell, J., and Miller, A.H., 1944, The distribution of the birds of California: Berkeley, Calif., Coper Ornithological Club, Pacific Coast Avifauna, no. 27, 608 p. 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., accessed September 19, 2022, at https://doi.org/10.3133/ofr20211053.

Howell, S.L. and Kus, B.E., 2009a, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2008 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 52 p. [Available at https://sdmmp.com/ view_article.php?cid=SDMMP_CID_184_665761e848c3c.] Howell, S.L. and Kus, B.E., 2009b, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2009 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 58 p. [Available at https://sdmmp.com/ view_article.php?cid=SDMMP_CID_187_659c2d7323a65.] Howell, S.L. and Kus, B.E., 2010, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2010 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 55 p. [Available at https://sdmmp.com/ view_article.php?cid=SDMMP_CID_187_659c31b8f377f.] Howell, S.L. and Kus, B.E., 2011, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2011 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 46 p. [Available at https://sdmmp.com/view_ article.php?cid=SDMMP_CID_187_659c336dabdd2.] Howell, S.L. and Kus, B.E., 2012, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2012 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 55 p. [Available at https://sdmmp.com/view_ article.php?cid=SDMMP_CID_187_659c346466ae0.] Howell, S.L. and Kus, B.E., 2013, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2013 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 56 p. [Available at https://sdmmp.com/view_ article.php?cid=SDMMP_CID_187_659c3583b50bd.] Howell, S.L. and Kus, B.E., 2014, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2014 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 59 p. [Available at https://sdmmp.com/view_ article.php?cid=SDMMP_CID_187_659c367d91e45.]

22 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher Howell, S.L. and Kus, B.E., 2015, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2015 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 89 p. [Available at https://sdmmp.com/view_ article.php?cid=SDMMP_CID_187_659c377866e97.] Howell, S.L. and Kus, B.E., 2016, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2016 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 87 p. [Available at https://sdmmp.com/view_ article.php?cid=SDMMP_CID_187_659c42972d23a.] Howell, S.L. and Kus, B.E., 2017, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2017 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 81 p. [Available at https://sdmmp.com/view_ article.php?cid=SDMMP_CID_187_659c45704d6fb]. Howell, S.L., and Kus, B.E., 2021, Distribution and abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the upper San Luis Rey River, San Diego County, California—2020 data summary: U.S. Geological Survey Data Series 1140, 11 p., accessed September 19, 2021, at https://doi.org/10.3133/ds1140. Howell, S.L., and Kus, B.E., 2024, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2020 annual report: U.S. Geological Survey Open-File Report 2024–1005, 35 p., https://doi.org/10.3133/ofr20241005. Hubbard, J.P., 1987, The status of the Willow Flycatcher in New Mexico: Santa Fe, N. Mex., New Mexico Department of Game and Fish, Endangered Species Program, 29 p. Kenwood, K.E. and Kus, B.E., 2007, Distribution, abundance and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2006 annual report: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 61 p. [Available at https://sdmmp.com/ view_article.php?cid=SDMMP_CID_184_65985edc6f1aa.] Kus, B.E., 2001, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California, in 2000—Final report: Prepared for the Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 39 p. [Available at https://sdmmp.com/ view_article.php?cid=CID_ctamanah%40usgs.gov_ 5751ba4d123be.]

Kus, B.E., Beck, P.P., and Wells, J.M., 2003, Southwestern Willow Flycatcher populations in California—Distribution, abundance, and potential for conservation: Studies in Avian Biology, v. 26, p. 12–21. Kus, B.E. and Ferree, K., 2003, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2001 annual report: Prepared for the Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 22 p. [Available at https://sdmmp.com/ view_article.php?cid=CID_ctamanah%40usgs.gov_ 5751bbad0c289.] Kus, B.E. and Kenwood, K.E., 2003, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2002 annual report: Prepared for the Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 27 p. [Available at https://sdmmp.com/ view_article.php?cid=CID_ctamanah%40usgs.gov_ 5751bc9b0060f.] Kus, B.E. and Kenwood, K.E., 2005, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2003 annual report: Prepared for the Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 53 p. [Available at https://sdmmp.com/ view_article.php?cid=SDMMP_CID_184_6657554122ddf.] Kus, B.E. and Kenwood, K.E., 2006a, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2004 annual report: Prepared for the Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 60 p. [Available at https://sdmmp.com/ view_article.php?cid=SDMMP_CID_184_65984dd42f59f.] Kus, B.E. and Kenwood, K.E., 2006b, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2005 annual report: Prepared for the Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton, 55 p. [Available at https://sdmmp.com/ view_article.php?cid=SDMMP_CID_184_6598544b3fee9.] Laake, J.L., 2013, RMARK—An R interface for analysis of capture-recapture data with MARK: Seattle, Wash., Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Processed Report 2013‒01, 25 p. Office of Water Resources, 2021, Lake O’Neill Station precipitation summary: Camp Pendleton, Calif., Prepared for the Assistant Chief of Staff, Facilities, Marine Corps Base Camp Pendleton.

References Cited 23 Paxton, E.H., Sogge, M.K., Durst, S.L., Theimer, T.C., and Hatten, J.R., 2007, The ecology of the Southwestern Willow Flycatcher in central Arizona—A 10-year synthesis report: U.S. Geological Survey Open-File Report 2007–1381, 143 p., accessed on September 19, 2022, at https://pubs.usgs.gov/of/2007/1381/of2007-1381.pdf. Peterson, B.L., Kus, B.E., and Deutschman, D.H., 2004, Determining nest predators of the Least Bell’s Vireo through point counts, tracking stations, and video photography: Journal of Field Ornithology, v. 75, no. 1, p. 89–95, accessed September 19, 2022, at https://doi.org/10.1648/0273-8570-75.1.89. R Core Team, 2021, R—A language and environment for statistical computing: Vienna, Austria, R Foundation for Statistical Computing, accessed November 5, 2021, at http://www.R-project.org. Remsen, J.V., Jr., 1978, Bird species of special concern in California: State of California, The Resources Agency, Department of Fish and Game, Wildlife Management Division Administrative Report 78–1, 67 p. Rourke, J.W., McCarthey, T.D., Davidson, R.F., and Santaniello, A.M., 1999, Southwestern Willow Flycatcher nest monitoring protocol: Phoenix, Ariz., Arizona Game and Fish Department, Nongame and Endangered Wildlife Program Technical Report 144, 34 p. Rourke, J.W., Howell, S.L., and Kus, B.E., 2008, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2007 annual data summary: Prepared for Assistant Chief of Staff, Environmental Security, Marine Corps Base Camp Pendleton. Schlorff, R.W., 1990, Report to the Fish and Game Commission—Status review of the Willow Flycatcher (Empidonax traillii) in California: State of California, The Resources Agency, Department of Fish and Game, Wildlife Management Division, Department Candidate Species Status Report 90–04, 23 p., accessed on September 19, 2022, at https://nrm.dfg.ca.gov/FileHandler.ashx? DocumentVersionID=68768. Schofield, L.N., Loffland, H.L., Siegel, R.B., Stermer, C.J., and Mathewson, H.A., 2018, Using conspecific broadcast for Willow Flycatcher restoration: Avian Conservation and Ecology, v. 13, no. 1, article 23, accessed September 19, 2022, at https://doi.org/10.5751/ACE-01216-130123. Smith, D.M., Finch, D.M., Gunning, C., Jemison, R., and Kelly, J.F., 2009, Post-wildfire recovery of riparian vegetation during a period of water scarcity in the southwestern USA: Fire Ecology, v. 5, p. 38–55, accessed August 15, 2023, at https://doi.org/10.4996/fireecology.0501038.

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., accessed September 19, 2022, at https://doi.org/10.3133/tm2A10. State of California, 2024, State and federally listed endangered and threatened animals of California: Sacramento, Calif., Natural Resources Agency, California Department of Fish and Wildlife, Biogeographic Data Branch, California Natural Diversity Database. [Available at https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID= 109405&inline.] Unitt, P., 1984, The birds of San Diego County: San Diego Society of Natural History, Memoir 13, 276 p. Unitt, P., 1987, Empidonax traillii extimus—An endangered subspecies: Western Birds, v. 18, p. 137–162. 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, 1995, Final rule determining endangered status for the Southwestern Willow Flycatcher: Federal Register, v. 60, no. 38, p. 10694–10715. U.S. Fish and Wildlife Service, 2006, 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. Wheelock, I.G., 1912, Birds of California—An introduction to more than three hundred common birds of the state and adjacent islands: Chicago, Ill., A.C. McClurg and Company. White, G.C., and Burnham, K.P., 1999, Program MARK—Survival estimation from populations of marked animals: Bird Study, v. 46, no. sup 1, p. S120–S139, accessed September 19, 2022, 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. Willett, G., 1912, Birds of the Pacific slope of southern California: Hollywood, Calif., Cooper Ornithological Club, Pacific Coast Avifauna, v. 7. Willett, G., 1933, A revised list of the birds of southwestern California: Los Angeles, Calif., Cooper Ornithological Club, Pacific Coast Avifauna, v. 21, 204 p.

24 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher

Appendix 1. Southwestern Willow Flycatcher Survey Areas at Marine Corps Base Camp Pendleton, California, 2021 117.35000

117.30000

33.4000

De Luz Creek/South

33.35000

Santa Margarita River/Above Hospital

Fallbrook Creek/Lake O’Neill La ke O’N eill

Santa Margarita River/Below Hospital West

EXPLANATION Survey area Core Non-core C

0 0

0.5 0.5

1 MILE 1 KILOMETER

Figure 1.1. Southwestern Willow Flycatcher (Empidonax traillii extimus) survey areas at Marine Corps Base Camp Pendleton, California, 2021: Santa Margarita River (upstream) and Fallbrook Creek.

Appendix 1 25 117.45000

117.40000

117.35000

Santa Margarita River/Stagecoach

33.30000

Santa Margarita River/Rifle Range Santa Margarita River/Air Station East

Santa Margarita River/Pump Road North

Santa Margarita River/Effluent Seep French Creek

Santa Margarita River/Pump Road South Santa Margarita River/Bell North

33.25000

Santa Margarita River/Bell South

0 0

0.5 0.5

1 MILE 1 KILOMETER

Figure 1.2. Southwestern Willow Flycatcher (Empidonax traillii extimus) survey areas at Marine Corps Base Camp Pendleton, California, 2021: Santa Margarita River (downstream) and French Creek.

26 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 117.45000

117.40000

33.40000

Las Flores Creek/Upper Las Flores North

33.35000

Las Flores Creek/Lower Las Flores North

EXPLANATION Survey area Core Non-core C

0 0

0.5 0.5

1 MILE 1 KILOMETER

Figure 1.3. Southwestern Willow Flycatcher (Empidonax traillii extimus) survey areas at Marine Corps Base Camp Pendleton, California, 2021: Las Flores Creek.

Appendix 1 27 117.60000

117.55000

San Mateo Creek/Upper San Mateo

San Mateo Creek/Lower San Mateo Bottom 33.40000

33.35000

EXPLANATION Survey area Core Non-core C

0 0

0.5 0.5

1 MILE 1 KILOMETER

Figure 1.4. Southwestern Willow Flycatcher (Empidonax traillii extimus) survey areas at Marine Corps Base Camp Pendleton, California, 2021: San Mateo Creek (downstream).

28 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 117.55000

117.50000

33.50000

San Mateo Creek/Upper San Mateo 33.45000

EXPLANATION Survey area Non-core C

0 0

0.5 0.5

1 MILE 1 KILOMETER

Figure 1.5. Southwestern Willow Flycatcher (Empidonax traillii extimus) survey areas at Marine Corps Base Camp Pendleton, California, 2021: San Mateo Creek (upstream).

Appendix 1 29 117.30000

117.25000

33.30000

Pilgrim Creek/South of Vandegrift

33.25000

0 0

0.5 0.5

1 MILE 1 KILOMETER

Figure 1.6. Southwestern Willow Flycatcher (Empidonax traillii extimus) survey areas at Marine Corps Base Camp Pendleton, California, 2021: Pilgrim Creek.

30 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher

Appendix 2. Locations of Willow Flycatchers at Marine Corps Base Camp Pendleton, California, 2021 117.35013

117.34034

117.33055

117.32076

117.31098

DS01F

33.34998

HP201F 33.33489

Lak e

OL01F

ll O’Nei EXPLANATION Survey area

33.31980

Core Non-core C Survey plot Conspecific playback

SG05F

Willow Flycatcher locations and identifier OL01F

0 0

0.5 0.5

Transient (subspecies unknown)

1 MILE 1 KILOMETER

Figure 2.1. Locations of Willow Flycatchers (Empidonax traillii) at Marine Corps Base Camp Pendleton, California, 2021: Santa Margarita River (upstream) and Fallbrook Creek. Refer to table 2 for more information about bird identifiers.

Appendix 2 31 117.38928

117.37949

117.3697

117.35992

117.35013

SG03F

SG04F

33.30471

SG01F

SG02F

RR03F RR01F

Artificial seep

RR02F

33.28962

Artificial seep

Artificial seep ETC

PR03F

HP501F

ES01F

33.27453

EXPLANATION Survey area Core

PR01F

Non-core C

PR02F

Survey plot Conspecific playback

HP601F

Control Willow Flycatcher locations and identifier

33.25944

ETC

HN701F

HP501F

0 0

0.5 0.5

Resident unpaired female Transient (subspecies unknown)

1 MILE 1 KILOMETER

Figure 2.2. Locations of Willow Flycatchers (Empidonax traillii) at Marine Corps Base Camp Pendleton, California, 2021: Santa Margarita River (downstream). Refer to table 2 for more information about bird identifiers.

32 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 117.59483

117.58504

117.57525

117.56546

117.55568

33.42543

33.41034

MB01F

MB02F

33.39525

EXPLANATION Survey area Core Willow Flycatcher locations and identifier MB02F 33.38016

0 0

0.5 0.5

Transient (subspecies unknown)

1 MILE 1 KILOMETER

Figure 2.3. Locations of Willow Flycatchers (Empidonax traillii) at Marine Corps Base Camp Pendleton, California, 2021: San Mateo Creek (downstream). Refer to table 2 for more information about bird identifiers.

Appendix 2 33 117.51652

117.50674

117.49695

117.48716

33.47070

MU01F

MU02F

33.45561

33.44052

EXPLANATION

33.42543

Survey area Non-core C Willow Flycatcher locations and identifier MU02F

0 0

0.5 0.5

Transient (subspecies unknown)

1 MILE 1 KILOMETER

Figure 2.4. Locations of Willow Flycatchers (Empidonax traillii) at Marine Corps Base Camp Pendleton, California, 2021: San Mateo Creek (upstream). Refer to table 2 for more information about bird identifiers.

34 Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher 117.31098

117.30119

117.2914

117.28161

33.30471

PS02F

33.28962

PS01F

33.27453

EXPLANATION Survey area Core Survey plot Conspecific playback Control Willow Flycatcher locations and identifier

33.25944

PS02F

0 0

0.5 0.5

Transient (subspecies unknown)

1 MILE 1 KILOMETER

Figure 2.5. Locations of Willow Flycatchers (Empidonax traillii) at Marine Corps Base Camp Pendleton, California, 2021: Pilgrim Creek. Refer to table 2 for more information about bird identifiers.

Appendix 3 35

Appendix 3. Southwestern Willow Flycatcher Territory Locations at Marine Corps Base Camp Pendleton, California, 2021 117.37222

Va n d e g r i f t B o

33.28333

ulevard

117.37500

Territory ETC Unpaired female 33.28056

EXPLANATION Southwestern Willow Flycatcher breeding territory Territory boundary Nest location METERS 0

Figure 3.1. Southwestern Willow Flycatcher (Empidonax traillii extimus) territories at Marine Corps Base Camp Pendleton, 2021: Pueblitos Breeding Area, California, Santa Margarita River.

100

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

Howell and Kus—Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher—OFR 2024–1039

ISSN 2331-1258 (online) https://doi.org/10.3133/ofr20241039

Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 Annual Report

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