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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
Ecosystems Mission Area—Species Management Research Program
Prepared in cooperation with Assistant Chief of Staff, Environmental Security, U.S. Marine Corps Base Camp Pendleton
By:  and 
https://doi.org/10.3133/ofr20241039

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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 (ryan.besser@usmc.mil) 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).

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.

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

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:

1. White labels describe drainages; blue labels describe historical breeding areas.
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).

Core Areas

  1. 1. Santa Margarita River:

    1. a. Above Hospital, Below Hospital West (app. 1, fig.1.1).

    2. b. Air Station East, Effluent Seep, Bell North, Bell South, Pump Road North, Rifle Range (app. 1, fig. 1.2).

  2. 2. Fallbrook Creek, Lake O’Neill (app. 1, fig. 1.1).

  3. 3. Las Flores Creek, Upper Las Flores North (app. 1, fig. 1.3).

  4. 4. San Mateo Creek, Lower San Mateo Bottom (app. 1, fig. 1.4).

  5. 5. Pilgrim Creek, South of Vandegrift (app. 1, fig. 1.6).

Rotating Non-Core Areas: C

  1. 1. Santa Margarita River, Pump Road South, Stagecoach (app. 1, fig. 1.2).

  2. 2. De Luz Creek, South (app. 1, fig. 1.1).

  3. 3. French Creek (fig. 1; app. 1, fig. 1.2).

  4. 4. Las Flores Creek, Lower Las Flores North (app. 1, fig. 1.3).

  5. 5. San Mateo Creek, Upper San Mateo (app. 1, fig. 1.5).

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).

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.

2. Overview of the study area with symbols for described features.
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.

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.

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).

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).

Table 1.    

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

[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 2 2 2 3 2 1 2 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0
LF/Las Flores 0 0 0 2 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
PC/Pilgrim 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0
SO/San Mateo 0 0 0 0 0 0 0 2 0 1 1 1 1 0 0 0 0 0 0 0 0 0
SR/Above Hospital 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
SR/Below Hospital 0 0 2 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
SR/Air Station 6 5 2 0 2 0 0 4 4 5 6 5 6 5 3 0 1 0 20 1 0 0
SR/Rifle Range 0 0 0 1 0 0 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0
SR/Pump Road 2 6 6 5 11 9 6 8 3 3 3 2 11 2 0 0 0 0 0 0 0 0
SR/Treatment Ponds 1 1 0 0 0 1 5 4 2 4 4 3 3 4 3 4 2 0 20 0 0 0
SR/Pueblitos 4 7 6 9 8 4 9 2 5 3 1 1 0 11 2 11 0 0 3 2 2 1
SR/Ysidora Ponds 5 8 4 4 6 9 5 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0
SR/Bell 4 4 6 3 10 5 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
SR/Vine 4 2 4 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
SR/Stuart Mesa 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Total 28 35 33 32 40 29 31 26 14 16 15 12 12 13 9 5 3 0 3 3 2 1
Table 1.    Distribution of territorial Southwestern Willow Flycatchers (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton, 2000–21.
1

One male's territory overlapped two breeding areas, included in Treatment Ponds total.

2

Two different females utilized additional breeding areas (Air Station and Treatment Ponds) before settling in Pueblitos, included in Pueblitos total.

3. Stacked bar graph showing the annual number of flycatchers, separated by sex.
Figure 3.

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

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).

Conspecific Playback Surveys

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.

4. Overview of the study area with polygons and symbols for described features.
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.

5. Overview of the study area with polygons and symbols for described features.
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.

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).

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 T Willow/sycamore 1 —
HP201F Fallbrook Creek T Mixed willow 1 —
OL01F Fallbrook Creek T Mixed willow 1 —
PS01F Pilgrim Creek T Mixed willow 1 —
PS02F Pilgrim Creek T Oak/sycamore 1 —
MB01F San Mateo Creek T Riparian scrub 2 BRA, CON
MB02F San Mateo Creek T Riparian scrub 2 BRA, CON
MU01F San Mateo Creek T Willow/sycamore 1 —
MU02F San Mateo Creek T Willow/sycamore 1 —
ES01F Santa Margarita River T Mixed willow 1 —
ETC Santa Margarita River S Mixed willow 2 CON
HN701F Santa Margarita River T Mixed willow 3 CON
HP501F Santa Margarita River T Mixed willow 3 CON
HP601F Santa Margarita River T Mixed willow 3 CON
PR01F Santa Margarita River T Mixed willow 1 —
PR02F Santa Margarita River T Mixed willow 1 —
PR03F Santa Margarita River T Mixed willow 1 BRA
RR01F Santa Margarita River T Riparian scrub 2 CON
RR02F Santa Margarita River T Non-native 3 EUC
RR03F Santa Margarita River T Mixed willow 2 CON
SG01F Santa Margarita River T Mixed willow 2 PIC
SG02F Santa Margarita River T Mixed willow 3 BRA
SG03F Santa Margarita River T Riparian scrub 3 BRA
SG04F Santa Margarita River T Mixed willow 1 CON
SG05F Santa Margarita River T Mixed willow 2 CON
Table 2.    Habitat characteristics of Willow Flycatcher (Empidonax traillii) locations at Marine Corps Base Camp Pendleton, 2021.

Breeding Productivity

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.

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		 Comments
ETC 05/22/2021 4 0 0 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 3 0 0 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.
Table 3.    Nesting activity of Southwestern Willow Flycatchers (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton, 2021.

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.

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.

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: PrecipTW, total winter precipitation; PrecipLW, late winter precipitation from January 1 to March 31 of the breeding season year; Precip EW, early winter precipitation from October 1 to December 31 of the calendar year before the breeding season; +, plus]
Model AICc ΔAICc AICc
weight		 Deviance Number of
parameters
Sex 50008.1 0.0 0.37 49773.3 4
PrecipTW 50009.1 0.9 0.23 49774.3 4
PrecipLW 50010.3 2.2 0.12 49775.5 4
PrecipEW 50010.4 2.3 0.12 49775.6 4
PrecipEW+PrecipLW 50011.1 2.9 0.08 49774.2 5
Constant 50011.2 3.1 0.08 49778.5 3
Sex+Year 50023.4 15.3 0.00 49744.9 24
Year 50027.6 19.4 0.00 49751.4 23
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.

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 Effect β SE Odds
ratio		 95-percent
CI
Sex Intercept1 0.7 0.2 2.0 1.41–2.92
Male −0.6 0.3 0.6 20.33–0.92
PrecipTW Intercept −0.1 0.3 0.9 0.49–1.63
PrecipTW 0.0 0.0 1.0 31.00–1.04
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.
1

Intercept includes females.

2

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

3

Value before rounding up was below 1.00.

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 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 4
Age+Year 860.3 19.4 0.00 282.2 24
Constant 889.4 48.5 0.00 355.4 3
PrecipEW 889.5 48.6 0.00 353.5 4
PrecipTW 890.5 49.5 0.00 354.5 4
PrecipLW 891.2 50.3 0.00 355.2 4
PrecipEW+PrecipLW 891.6 50.6 0.00 353.5 5
Year 908.0 67.1 0.00 332.1 23
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.

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 β SE Odds
ratio		 95-percent
CI
Age Intercept −1.4 0.2 0.2 0.17–0.35
Adult 1.8 0.2 6.0 13.84–9.37
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.
1

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

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.

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.

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. 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. 2. Implementing habitat restoration in previously burned (Pump Road) breeding areas.

  3. 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.

  4. 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]):

    • • Settling ponds (33.28351, −117.37373; 33.26235, −117.37162)

    • • Canal/ditch (33.28254, −117.37361 [initiated in 2018]; 33.28053, −117.37371; 33.27817, −117.37476; 33.25122, −117.37616)

    • • 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

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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://doi.org/10.3133/ofr20081303.

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

1.1. Overview of the study area with yellow polygons indicating core survey areas and green polygons representing non-core survey areas.
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.

1.2. Overview of the study area with yellow polygons indicating core survey area and green polygons representing non-core survey areas.
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.

1.3. Overview of the study area with yellow polygons indicating core survey area and green polygons representing non-core survey areas.
Figure 1.3.

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

1.4. Overview of the study area with yellow polygons indicating core survey area and green polygons representing non-core survey areas.
Figure 1.4.

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

1.5. Overview of the study area with green polygons indicating non-core survey areas.
Figure 1.5.

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

1.6. Overview of the study area with yellow polygons indicating core survey areas.
Figure 1.6.

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

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

2.1. Overview of the study area with core survey areas shown in yellow, non-core survey areas shown in green, one conspecific playback plot shown as a green circle, and transient flycatchers shown as yellow diamonds.
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.

2.2. Overview of the study area with core survey areas shown in yellow and non-core survey areas shown in green. Multiple conspecific playback plots (green) and control plots (red) are located along the length of this river reach. One resident flycatcher (orange square) was detected near a playback plot. Transient flycatchers (yellow diamonds) occurred near both playback and control plots.
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.

2.3. Overview of the study area with core survey areas shown in yellow. Only two transients (yellow diamonds) were detected in this survey area.
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.

2.4 Overview of the study area with non-core survey areas shown in green. Only two transients (yellow diamonds) were detected in this survey area.
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.

2.5. Overview of the study area with core survey areas shown in yellow, one conspecific playback plot shown as a green circle, and one control plot shown as a red circle. Only two transients (yellow diamonds) were detected in the survey area.
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. Southwestern Willow Flycatcher Territory Locations at Marine Corps Base Camp Pendleton, California, 2021

3.1. Overview of the study area showing the territory boundary and territory ID of an unpaired female flycatcher.
Figure 3.1.

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

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain
centimeter (cm) 0.3937 inch (in.)
meter (m) 3.281 foot (ft)
kilometer (km) 0.6214 mile (mi)
hectare (ha) 2.471 acre
hectare (ha) 0.003861 square mile (mi2)
kilometer per hour (km/hr) 0.6214 mile per hour (mi/hr)

Datums

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

min

minutes

s

seconds

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

Disclaimers

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)

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 annual report
Series title Open-File Report
Series number 2024-1039
DOI 10.3133/ofr20241039
Year Published 2024
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description viii, 35 p.
Country United States
State California
Other Geospatial Marine Corps Base Camp Pendleton
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details