Introduction

The purpose of this report is to provide the Marine Corps with an annual summary of the distribution, abundance, and breeding activity of the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus; flycatcher) at Marine Corps Base Camp Pendleton (MCBCP or “Base”). The results are intended to provide the Base with biological information to inform appropriate management of the flycatcher and support the dual missions of environmental stewardship and maintaining military readiness in accordance with the Base Integrated Natural Resources Management Plan (Marine Corps Base Camp Pendleton, 2023) and U.S. Fish and Wildlife Service Programmatic Biological Opinion (U.S. Fish and Wildlife Service, 1995).

The Southwestern Willow Flycatcher is one of four subspecies of Willow Flycatcher in the United States, with a breeding range that includes southern California, Arizona, New Mexico, extreme southern parts of Nevada 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 Southwestern Willow Flycatcher was listed as endangered by the State of California in 1992 and by the U.S. Fish and Wildlife Service in 1995. After listing, population estimates for flycatchers in California increased to 256 territories, with the increase largely attributed to expanded survey efforts rather than population growth at known sites (U.S. Fish and Wildlife Service, 2002). In the 2014 5-year status review, estimates of California flycatcher territories decreased to 172, with declines occurring statewide (Durst and others, 2008; U.S. Fish and Wildlife Service, 2014).

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. As of 2023, most of the Southwestern Willow Flycatchers in California are concentrated at two known sites: (1) the Owens River valley in Inyo County (approximately 57 territories; Great Basin Bird Observatory, 2023) and (2) the upper San Luis Rey River at Lake Henshaw in San Diego County (approximately 51 territories; Howell and Kus, 2024d). Outside of these sites, Southwestern Willow Flycatchers occur as small, isolated populations of one to a half-dozen pairs. Many of these small populations in San Diego County have been occupied intermittently, including Bonsall, Couser Canyon, Guajome Lake, and Whelan Lake on the San Luis Rey River, San Dieguito River, San Diego River, and Sweetwater River (Unitt, 1987; Kus and others, 2003). The three locations closest to MCBCP, Bonsall, Guajome Lake, and Whelan Lake, were last occupied in 2021, 2005, and 2007, respectively (B. Kus, unpub. data, 2005; Allen and Kus, 2022; Houston and others, 2024).

Male Southwestern Willow Flycatchers typically begin arriving in southern California in early to mid-May, whereas females arrive approximately 1 week later. Territorial males sing repeatedly from exposed perches while on the breeding grounds in order to attract a mate. Once a pair bond is established, or in some cases without the presence of a male, the female builds an open-cup nest that usually is placed in a branch fork of a willow (Salix spp.) or plant with a similar branching structure, approximately 1–3 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 and early September to their wintering grounds in central Mexico, Central America, and northern South America.

The population of Southwestern Willow Flycatchers at 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 playback study was initiated in 2018 to explore if this method 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 with Willow Flycatchers of the adastus and brewsteri subspecies (Schofield and others, 2018), but it has not been applied to any populations of the extimus subspecies. This report presents a preliminary evaluation of Southwestern Willow Flycatcher response to conspecific playback.

In addition, three artificial seeps were installed in historically occupied flycatcher habitat for the purpose of habitat enhancement. The first seep began operation in 2019, and two additional seeps were added in 2021. The seeps were designed to augment surface water and enhance habitat for breeding flycatchers. Although this enhancement was designed to benefit flycatchers, few flycatchers have inhabited the habitat enhancement areas in recent years. However, Least Bell’s Vireos are abundant in the enhancement areas, and were selected as a surrogate species to determine the effects of the habitat enhancement. Vireos co-occur with flycatchers in riparian habitat and have similar habitat requirements, such as the presence of riparian obligate trees (typically willows and cottonwoods) with a shrubby understory. Vireos and flycatchers have similar territory size and territorial behavior (singing from high perches to advertise territory boundaries), and they share some similarities in nest placement (nests are placed in the understory vegetation). Although there are some differences in habitat requirements between the two species (flycatchers prefer more mesic conditions that include surface water or elevated soil moisture during at least part of the breeding season; vireos are more tolerant of drier, brushier vegetation sometimes lacking an overstory), vireos were considered sufficiently similar to flycatchers to serve as a surrogate species for analysis.

The purpose of this study, which began in 2000, was to document the status of Southwestern Willow Flycatchers at 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 annual survival, fidelity, 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. Data from 2024, combined with data from 2000 to 2023, 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 ensure the species’ continued existence.

This work was funded by and 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 conducted under federal 10(a)1(A) Recovery Permit ESPER0004080_0.2 and a protocol approved by the Western Ecological Research Center (WERC) Institutional Animal Care and Use Committee for conformance with the Animal Welfare Act.

Study Areas and Methods

Population Size and Distribution

From 2000 to 2018, all of MCBCP’s major drainages and several minor ones that supported riparian habitat were surveyed annually for flycatchers. In 2019, a reduced monitoring plan was implemented in which annual surveys for flycatchers were done 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 to be surveyed on a rotational schedule every 5 years, beginning in 2020. In 2024, all of MCBCP’s historically occupied riparian habitat (core survey area; fig. 1) was surveyed for flycatchers. Because of funding limitations, none of the non-core survey groups were surveyed for flycatchers in 2022, 2023, or 2024. Protocol surveys were done three times between May 15 and July 31 (fig. 1; app. 1, figs. 1.1–1.5). Field work was completed by U.S. Geological Survey personnel: Lisa Allen, Annabelle Bernabe, Alexandra Houston, Scarlett Howell, Suellen Lynn, Jessica Medina, Shannon Mendia, and Maia Nguyen. The specific areas surveyed are listed in the following section.

Core Areas

1. 1. Santa Margarita River:

   1. a. Air Station East, Effluent Seep, Bell North, Bell South: from Basilone Road to a point approximately 8.5 kilometers (km) downstream on the east side of the Santa Margarita River (app. 1, fig. 1.2).

   2. b. Rifle Range, Pump Road North: from the Rifle Range along Stagecoach Road to a point approximately 2.5 km downstream (app. 1, fig. 1.2).

   3. c. Above Hospital, Below Hospital West: from the confluence with De Luz Creek to Basilone Road (app. 1, fig. 1.1).

2. 2. Fallbrook Creek, Lake O’Neill: at the inflow to Lake O’Neill, as well as around the lake (app. 1, fig. 1.1).

3. 3. Las Flores Creek, Upper Las Flores North: between a point 1.6 km downstream from Basilone Road to the Zulu Impact Area boundary approximately 0.8 km upstream from Basilone Road, including the side drainage adjacent to the 43 area (app. 1, fig. 1.3).

4. 4. San Mateo Creek, Lower San Mateo Bottom: between the Pacific Ocean and a point approximately 3.6 km upstream, including habitat south of the creek and south of the agricultural fields (app. 1, fig. 1.4).

5. 5. Pilgrim Creek, South of Vandegrift: between the southern Base boundary and Vandegrift Boulevard, including the two side drainages east of Pilgrim Creek (app. 1, fig. 1.5).

Investigators followed standard survey protocol (Sogge and others, 2010), moving slowly (approximately 2 km per hour) 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 flycatchers were not 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) and 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 and 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 song (fitz-bew) 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 min). 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, unpaired, or transient), and if possible, if the bird was banded. Flycatcher locations were mapped using Esri Field Maps (Esri, 2024) on Samsung Galaxy XCover6 Pro mobile phones with Android operating systems and built-in Global Positioning Systems to determine geographic coordinates (World Geodetic System 1984).

Conspecific Playback Surveys

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 on the Santa Margarita River, one was at Lake O’Neill, and one was at Pilgrim Creek. Six of the control plots were on the Santa Margarita River, and the remaining plot was 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 30 to August 1, 2024, 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.

Surveys were done every other week from May to July in the conspecific playback and control plots to determine if any Southwestern Willow Flycatchers had established a territory. Surveys in 2024 began on May 8 and concluded on July 24. Investigators surveyed within 50 m of all historical breeding territories that fell within the playback or control plot, following a slightly modified survey protocol developed to attract Willow Flycatchers of the adastus and brewsteri subspecies 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 flycatchers were not detected during the initial listening period, investigators broadcasted the Willow Flycatcher song for approximately 30 s and 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 flycatchers were not 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, if the survey was done during the time the broadcast was scheduled, the units were turned off before beginning the survey.

Results

Population Size and Distribution

Transients

Three Willow Flycatchers of unknown subspecies were observed during protocol and conspecific playback surveys in 2024 (app. 2, figs. 2.1–2.3). All transients were detected between May 16 and June 5. Transients were detected on the Santa Margarita River and San Mateo Creek, but were not detected on Fallbrook, Las Flores, or Pilgrim Creeks.

Residents

One Southwestern Willow Flycatcher (female) was detected during the 2024 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 males were observed in 2024. Overall, the resident flycatcher population on Base was the same as in 2023 (fig. 3; table 1).

Table 1.    

Distribution of territorial Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton, 2000–24.

[Refer to fig. 1 for drainage/breeding areas. Drainage/breeding area: FC, Fallbrook Creek; LF, Las Flores Creek; PC, Pilgrim Creek; SO, San Mateo Creek; SR, Santa Margarita River]

Table 1.    Distribution of territorial Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton, 2000–24.

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	2022	2023	2024
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	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	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	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	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	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	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	0	1	1
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	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	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	0	0	0
SR/Pueblitos	4	7	6	9	8	4	9	2	5	3	11	1	0	11	2	11	0	0	3	2	2	1	0	0	0
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	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	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	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	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	0	1	1

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.

In 2024, only the Air Station flycatcher breeding area along the Santa Margarita River was occupied. All other breeding areas along the Santa Margarita River that historically supported resident flycatchers (Vine, Bell, Ysidora Ponds, Pump Road, Pueblitos, and Treatment Ponds; fig. 1) were devoid of flycatcher territories in 2024 (table 1).

Discussion

In 2024, one unpaired female Southwestern Willow Flycatcher was detected on MCBCP, the same as in 2023. The Southwestern Willow Flycatcher population at MCBCP remains in a critical state, having 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 by more than 24 percent from one 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). Resident flycatchers were not detected on Base in 2017, but in 2018, three flycatchers that were previously detected on Base returned and resumed breeding. The resident population remained at three individuals from 2018 to 2019 before resuming the downward trend, with a 67-percent decline from 2019 to 2020, followed by a 50-percent decline from 2020 to 2021 when no breeding male flycatchers were detected on Base, leaving a single resident female that was not detected in 2022. After no resident flycatchers were observed in 2022, a natal female established a territory in 2023 but remained unpaired throughout the breeding season.

The number of transient Willow Flycatchers detected annually in the core survey area has varied greatly, despite consistent survey scope and effort, from a low of 2 in 2000 to a high of 42 in 2016. Although factors influencing the migratory route and variable timing of transient Willow Flycatchers are unclear, it remains clear that MCBCP provides important stop-over habitat for migrating Willow Flycatchers. Transient flycatchers were observed along two drainages on Base in 2024, including the Santa Margarita River and San Mateo Creek. Transient flycatchers in 2024 were all found in riparian scrub habitat, compared to the one resident flycatcher who occupied habitat with a wider variety of willow species. Exotic vegetation was dominant at multiple transient locations in 2024.

In the past 5 years, the small number of resident flycatchers on Base have occupied either the Air Station or Pueblitos breeding area, and only the Air Station breeding area has been occupied since 2023. In 2024, the same female flycatcher that was present in 2023 returned to her previous breeding area and territory and made three nesting attempts, but no male flycatcher was detected and breeding was ultimately unsuccessful. Prior to 2023, 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. The Air Station breeding area has not been occupied by a male flycatcher since the fire; however, an unpaired breeding female attempted to nest in the area in 2016 and 2018, building a nest and laying presumably infertile eggs both years. In 2018, the breeding female present in the Air Station breeding area remained in the area for approximately 1 month before moving to the Pueblitos breeding area to pair with a male in that area. In 2019, the same breeding female returned to the Air Station breeding area and successfully fledged young. In 2020, 2021, and 2022, breeding flycatchers were not detected in the Air Station breeding area. The Pueblitos breeding area has been the most consistently occupied breeding area on Base, supporting up to six pairs annually in 19 of the 25 years since annual monitoring began in 2000. Beginning in 2009, occupancy fluctuated between zero and one pair, with the exception of 2018 when two pairs occupied the area. In 2019 and 2020, Pueblitos was occupied by a single pair, and in 2021, Pueblitos was occupied only by an unpaired female. Resident flycatchers were not detected in the Pueblitos breeding area in 2022, 2023, or 2024.

Over the past two decades, many factors have been suggested as potential reasons for the decline among the remaining historically occupied breeding areas, including habitat senescence, overgrowth of exotic vegetation, habitat changes related to drought and changes in surface water, habitat loss from fire, and invasive insects. Detailed discussion on the potential reasons flycatchers may have moved out of all other breeding areas can be found in previous reports (Howell and Kus, 2024a, b, c). Habitat declines resulting from invasive the shot hole borer, which was first noted in 2023, continued in 2024. Large stands of trees, including willow and sycamore along the Santa Margarita River, appeared to be infested and were declining in health or dead. The canopy appeared to be less dense in many historically occupied breeding areas, and there were many dead trees with the tops snapped off. Although some willow trees that appeared to have been affected by shot hole borer in past years were sprouting new growth and may eventually recover, the overall character of the area has changed, which may reduce suitability for breeding flycatchers in the short term.

The primary obstacle that must be overcome in order for the MCBCP flycatcher population to rebound is low population size. Because there has been no productivity from the breeding flycatchers on MCBCP since 2020, there is no additional recruitment expected in the coming years. In order for the population to rebound, immigration will have to occur. In past years, our banding studies have allowed us to document both immigration into and emigration out of the MCBCP population, providing clear evidence that MCBCP played a role in the regional metapopulation. Immigration from nearby populations on the San Luis Rey River occurred multiple times in the early years of the study (2002, 2004, 2006–08), with adult and first-year flycatchers moving onto Base. However, populations along the San Luis Rey River that once augmented the MCBCP population, including Guajome Lake, Whelan Lake, and Bonsall, were extirpated in 2006, 2008, and 2021, respectively, and there is no longer a close source of potential immigrants (B. Kus, U.S. Geological Survey, unpub. data, 2006; Allen and Kus, 2022; Houston and others, 2024). The closest known population of Southwestern Willow Flycatchers in San Diego County is at Lake Henshaw on the upper San Luis Rey River, approximately 60 km from MCBCP. The upper San Luis Rey River population has been monitored annually since 2015, and during that time, only one MCBCP natal has been detected in the area (Howell and Kus, 2022; Howell and others, 2022). Although long distance dispersal can occur, most of the movements documented on MCBCP and other locations in San Diego County have been shorter dispersals (Howell and others, 2022).

Although the drivers of the decline of resident flycatchers on MCBCP remain unclear, several measures have been initiated in recent years in an attempt to reverse the declines, including the use of conspecific attraction to facilitate recolonization of historically occupied breeding areas. An experimental study using conspecific playback was initiated on Base in 2018 and has continued annually through 2024 (Howell and others, 2018, 2020; Howell and Kus, 2024a, b, c). In 2018, when the study on Base was first initiated, a previously unknown male settled adjacent to the playback, allowing breeding activities to resume after no breeding flycatchers were detected on Base in 2017. This male returned for two additional breeding seasons (2019, 2020) but was not present in 2021 or 2022. In 2023, a natal female settled within 110 m of the conspecific attraction playback in the Air Station breeding area, but remained unpaired. This female returned to the same area in 2024, and once again, remained unpaired. It is possible that conspecific attraction will be effective in attracting a male to this area in future years. The continuation of conspecific attraction playback may play an important role in restoring the population of resident flycatchers on Base.

The seep habitat enhancement project initiated by AC/S Environmental Security to augment surface water in historically occupied habitat is another important step in attempting to mitigate declines in the flycatcher population that could be related to changes in surface water on Base. In 2024, the nesting female placed her first nest approximately 90 m from the closest seep output in the Air Station breeding area, where she nested in 2023. However, her second nesting attempt was placed within 5 m of the surface water pool provided by the seep output, and the female was observed foraging and catching aerial insects among the vegetation surrounding the water pool (S. Howell, pers. obs., 2024). Additional surface water provided by the seeps may increase food resources, which in turn, could increase fitness and body condition. The addition of surface water may also assist in creating desirable habitat conditions for breeding flycatchers, such as a dense understory from 0 to 3 m, which was reported to be an important variable in previous nest vegetation studies (Rourke and others, 2004). Recreating these conditions may prompt flycatchers to establish territories in the enhanced areas in future years. A combination of surface-water enhancement and the related benefits to breeding habitat, and conspecific broadcasts in enhanced areas could be key to restoring the Southwestern Willow Flycatcher population on MCBCP.

Conclusions

The Southwestern Willow Flycatcher population in California seems to be experiencing a statewide decline that is not isolated to Marine Corps Base Camp Pendleton (MCBCP or “Base”). Populations on the Kern River (Mary Whitfield, Southern Sierra Research Station, written commun., 2023), Bonsall on the San Luis Rey River (Allen and Kus, 2022), and the lower San Luis Rey River (Houston and others, 2024) have experienced steep declines or have been extirpated in recent years. The one exception within the San Diego region appears to be the upper San Luis Rey River population at Lake Henshaw, which has experienced rapid growth since 2018, increasing to 74 individuals by 2023 (Howell and Kus, 2024d). After a high of 42 flycatchers in 2004, the population at MCBCP has been on a downward trajectory from 2005 to the present (2024), with temporary extirpations occurring in 2017 and 2022. Although one resident Willow Flycatcher returned to the Base in 2023, and was present again in 2024, the population continues to be at a critical low. Habitat loss from fire, potential changes in habitat composition, prolonged drought, and invasive insects have likely been detrimental to the long-term persistence of this endangered species on Base. Southwestern Willow Flycatchers also may be facing pressures on their wintering grounds, including, but not limited to, habitat degradation and conversion to agriculture (M. Whitfield, Southern Sierra Research Station, written commun., 2020).

Our long-term research supports the environmental stewardship program at MCBCP to protect endangered species and promote their recovery. Based on our observations of Southwestern Willow Flycatchers breeding under a variety of environmental conditions, the following actions have high potential for enhancing habitat suitability and availability on Base, thereby contributing to flycatcher recovery:

Until the Southwestern Willow Flycatcher population on Base increases, any projects that alter the habitat in current and historically occupied areas warrant careful consideration. In addition, communication between the AC/S Environmental Security and other military departments will become increasingly important. Our findings and experience indicate that effects to flycatcher habitat 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 among the sometimes competing land uses on Base, including military activities, recreation, habitat protection, and endangered species management.