Introduction

The Southwestern Willow Flycatcher (Empidonax traillii extimus, flycatcher) is one of four subspecies of Willow Flycatcher in the United States, with a breeding range that includes southern California, Arizona, New Mexico, extreme southern parts of Nevada 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.

Southwestern Willow Flycatchers in southern California co-occur with the Least Bell’s Vireo (Vireo bellii pusillus; hereafter “vireo”), which is 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 2021, the majority of Southwestern Willow Flycatchers in California are concentrated at two known sites: (1) the Owens River valley in Inyo County (M. Whitfield, Southern Sierra Research Station, written commun., 2021) and (2) the upper San Luis Rey River at Lake Henshaw in San Diego County (Howell and Kus, 2022). Outside of these sites, Southwestern Willow Flycatchers exist 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 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 in early to mid-May, whereas females arrive approximately 1 week later. Territorial males sing repeatedly from exposed perches while on the breeding grounds. Once the pair bond is established, the female builds an open-cup nest that is 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 central Mexico, Central America, and northern South America.

The population of Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton (MCBCP or “Base”) 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 was operated 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 brushy understory. Vireos and flycatchers have similar territory size, similar 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 these 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 (Lynn and others, 2024).

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 survival (including modeling factors that might affect survival, such as sex, age, and precipitation), 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. These data, combined with data from 2000 to 2021, 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 the Assistant Chief of Staff (AC/S), Environmental Security, Resources Management Division, Marine Corps Base Camp Pendleton, California. All activities were conducted under 10(a)1(A) Recovery Permit ESPER0004080_0.1.

Study Areas and Methods

Population Size and Distribution

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 2022, all of MCBCP’s historically occupied riparian habitat (core survey area; fig. 1) was surveyed for Willow Flycatchers. Because of funding limitations, none of the non-core survey groups were surveyed for flycatchers in 2022. Protocol surveys were done three times between May 15 and July 31 (fig. 1; appendix 1, figs. 1.1–1.5). Field work was completed by U.S. Geological Survey personnel: Lisa Allen, Annabelle Bernabe, Christopher Aaron Gallagher, Alexandra Houston, Scarlett Howell, Suellen Lynn, Devin Taylor, and Michelle Treadwell. The specific areas surveyed are listed here:

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 (appendix 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 (appendix 1, fig. 1.2).

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

2. 2. Fallbrook Creek, Lake O’Neill: at the inflow to Lake O’Neill, as well as around the lake (appendix 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 (appendix 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 (appendix 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 (appendix 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 minutes). Flycatchers that did not sing by the end of the encounter but were suspected to be breeding were revisited within 3 days (see “Breeding Productivity” in the “Study Areas and 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, if the bird was banded. Flycatcher locations were mapped using ESRI Field Maps (Environmental Systems Research Institute, 2022) on Samsung Galaxy S7 and S8 and LG G5 mobile phones with Android operating systems and built-in Global Positioning System to determine geographic coordinates (World Geodetic System 1984 [WGS 84]).

Conspecific Playback Surveys

Historic 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 May 2 to August 31, 2022, 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 2022 began on May 7 and concluded on July 20. Investigators surveyed within 50 m of all historic breeding territories that fell within the playback or control plot, following a slightly modified survey protocol developed to attract 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 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 historic 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 prior to the survey.

Results

Population Size and Distribution

Transients

Eight Willow Flycatchers of unknown subspecies were observed during protocol and conspecific playback surveys (appendix 2, figs. 2.1–2.3). All transients were detected between May 17 and May 25. Transients were detected on three of the five drainages surveyed in 2022. Willow Flycatchers were not detected on Fallbrook or Pilgrim Creeks.

Residents

Resident Southwestern Willow Flycatchers were not detected in 2022. Overall, the resident flycatcher population on Base decreased by 100 percent (one individual to zero individuals) from 2021 to 2022 (fig. 3; table 1).

Table 1.    

Distribution of territorial Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton, 2000–22.

[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, 2000–22.

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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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

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.

Discussion

In 2022, breeding Southwestern Willow Flycatchers were not detected on MCBCP, after only one unpaired female was observed in 2021. From 2000 to 2004, the resident population of Southwestern Willow Flycatchers on Base increased annually, culminating in a 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). 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.

The number of transients 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 multiple drainages on Base, including the Santa Margarita River and Las Flores and San Mateo Creeks. Transient flycatchers used multiple habitat types, including mixed willow, riparian scrub, and upland scrub habitats. Exotic vegetation was dominant in multiple transient locations.

Many of the historic breeding areas along the Santa Margarita River have been occupied by multiple breeding pairs in multiple years: Air Station, Pump Road, Treatment Ponds, Pueblitos, Ysidora, Bell, and Vine (see historic breeding areas; fig. 1). Of these seven primary breeding areas (those that supported multiple breeding pairs in multiple years), only the Pueblitos breeding area has been occupied in recent years; however, it was devoid of flycatchers in 2022. The Pueblitos breeding area has been occupied by breeding birds in most years since 2000, supporting up to six pairs annually; although, since 2009, occupancy has 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 only occupied by an unpaired female. The reasons for the decline in Pueblitos are unclear; however, it is possible that the habitat may have reached a stage of ecological succession that is unsuitable for breeding flycatchers because this area has not experienced any disturbance since giant reed removal occurred in 2001. Flycatchers often occupy early successional habitat, which was historically created by natural processes, such as annual flooding that scoured the floodplain and created a mosaic of different aged stands of habitat (Theimer and others, 2018; U.S. Fish and Wildlife Service, 2002). Because natural processes such as flooding have been largely interrupted in modern times, disturbances such as vegetation removal can replace flooding and be used to create early successional habitat.

All other breeding areas more recently occupied by Southwestern Willow Flycatchers on MCBCP remained vacant in 2022, including the Treatment Ponds breeding area. The Treatment Ponds 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 Base), and was occupied by one female at the beginning of the 2018 breeding season before she moved to Pueblitos. As with Pueblitos, the reasons for decline in the Treatment Ponds and all other historical breeding areas on Base are unclear, but may include habitat senescence, changes in vegetation and reduced soil moisture related to long-term drought, or hydrologic changes.

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. 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 in the area 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, 2021, and 2022, breeding flycatchers were not detected in the Air Station breeding area. The Pump Road breeding area also was affected by the 2014 Las Pulgas wildfire and was last occupied before the fire, in 2013. 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.

Although the habitat seems to have recovered to pre-fire condition at the Air Station and Pump Road breeding areas, both remain unoccupied. 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 was not 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).

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. All the flycatcher breeding areas upstream from these three areas experienced some form of exotic vegetation removal between 1996 and 2002, but the overgrowth of exotic vegetation persisted in Vine, Bell, and Ysidora Ponds. 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 Base after exotic vegetation removal within 5–7 years: exotic vegetation was removed from the Pump Road area in 1996, which supported multiple breeding pairs by 2001; the Air Station area had exotic vegetation removed in 2000 and had returned to pre-removal occupancy by 2007; and the Treatment Ponds area had exotic vegetation removed in 2001, with recolonization by flycatchers by 2006. Mean (±SD) annual precipitation for the first 5 years after exotics removal at 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, 2022). 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 fell in more recent years (2017, 2019, and 2020), providing some relief from the drought, the 2018, 2021, and 2022 bio-years produced less than average precipitation, perpetuating drought conditions that did not favor vegetation growth.

Although we speculate that insufficient quantity and quality of breeding habitat may have contributed to the flycatcher decline on Base, a flycatcher habitat suitability model developed by U.S. Geological Survey biogeographer James Hatten predicted that there was still plenty of suitable habitat at MCBCP in 2022 (Hatten and Paradzick, 2003; Hatten, 2016; Hatten, 2022). Despite this prediction, flycatchers on Base have continued to decline, ultimately, for unknown reasons.

Annual survival of adults does not appear to be a factor contributing to the declines seen in the flycatcher population on Base. Survival estimates for adults were within the range of survival estimates reported in other long-term studies of flycatchers. Adult survival at MCBCP averaged 60±3 percent across years, which is slightly below survival estimates for adults at Roosevelt Dam in Arizona from 1996 to 2005 (64 percent; Paxton and others, 2007) and higher than the adult return rate reported at the Kern River from 1989 to 2007 (52±18 percent; Whitfield and Henneman, 2009). In 2022, sex (male or female) did not appear to influence adult survival like it had in previous years, which may be an effect of small sample sizes in recent years combined with the loss of the last remaining female in the population. Year also did not appear to influence adult annual survival, nor did we find an effect of precipitation on survival when modelling adults only or adults plus first-year birds.

Overall, first-year survival was lower than adult survival, which is typical for most passerines. Across all years, first-year survival averaged 20±3 percent. In other flycatcher populations, first-year survival estimates have been roughly half those of adult survival (Paxton and others, 2007; Howell and others, 2022), but first-year survival was on average only 33 percent of adult estimates at MCBCP. This reduction is likely a reflection that MCBCP fledglings have not recruited into the Base breeding population since 2013. From 2001 to 2012, from one to six MCBCP natal birds annually would recruit into their natal site (MCBCP) and establish their first breeding territory. Since 2013, juveniles that hatched at MCBCP have not returned in subsequent years to breed anywhere on Base. The reasons for the decline are unclear, but it is possible that first-year flycatchers since 2013 encountered habitat conditions that were unsuitable, 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 up to 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; the birds simply dispersed elsewhere. The lack of recruitment to MCBCP may be a result of low first-year survival, young birds dispersing to areas outside the Base, or both. Although the small number of fledglings produced on Base between 2013 and 2020 may have contributed 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, likely contributed to local extirpation in 2022.

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 (2004, 2006–08), with adult and first-year flycatchers moving onto Base. However, the nearby populations along the San Luis Rey River that once augmented the MCBCP populations are now defunct, and there is no longer a close source of potential immigrants. 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. 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 using 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 2022 (Howell and others, 2018, 2020; B. Kus, U.S. Geological Survey, unpub. data, 2021). 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. Although breeding flycatchers did not recolonize historically occupied areas on Base in 2022, it is possible that conspecific attraction will be effective 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 2022, flycatchers were not observed using the area adjacent to the seeps; however, the additional surface water provided by the seeps is likely to have future effects on the riparian habitat, especially in drought years. Increasing surface water may decrease the severity of drought effects to vegetation and food resources in years with below average rainfall, such as 2022 (24.6 cm: Office of Water Resources, 2022). 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; Howell and others, 2018). 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 subsequently conducting conspecific broadcasts in enhanced areas could be the 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., 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. After experiencing a high of 42 individuals in 2004, the population at MCBCP has been on a downward trajectory from 2005 to the present (2022). At least temporarily, the breeding population on MCBCP seems to have succumbed to the combined forces of prolonged drought, habitat loss from fire, slow recovery from exotic vegetation removal, and possibly habitat senescence. 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). This is the second time in recent years that resident flycatchers were not detected on Base; however, there is a possibility that this will also be a temporary extirpation, and breeding birds will return in future years. Until that time, any projects that alter the habitat in historically occupied areas warrant careful consideration. Restoration activities that create pockets of early successional habitat could improve the chances for recolonization by Southwestern Willow Flycatchers. The creation or restoration of water sources, such as holding ponds on Base near historically occupied areas, also may promote the return of Southwestern Willow Flycatchers, enhancing recovery of flycatchers on Base and in the region.

Based on our long-term 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 and thereby contributing to flycatcher recovery:

With the decline of Southwestern Willow Flycatchers on Base, 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.