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Distribution and Abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Upper San Luis Rey River, San Diego County, California—2022 Data Summary

Data Report 1173
Ecosystems Mission Area—Species Management Research Program
By:  and 
https://doi.org/10.3133/dr1173

Links

Acknowledgments

This research was funded by the San Diego Association of Governments (SANDAG). The authors thank the land managers, property owners, and other stakeholders who facilitated access to the survey locations, including but not limited to: Kirsten Winter (U.S. Forest Service); Board of Directors, Richard Larsen, and Don Smith (Vista Irrigation District); Rey River Ranch; and Will Wilson.

The authors would like to thank Suellen Lynn and Devin Taylor (U.S. Geological Survey), who assisted in the data collection for this project.

Executive Summary

We surveyed for Southwestern Willow Flycatchers (Empidonax traillii extimus; flycatcher) along the upper San Luis Rey River near Lake Henshaw in Santa Ysabel, California, in 2022. Surveys were completed at four locations: three downstream from Lake Henshaw, where surveys occurred from 2015 to 2021 (Rey River Ranch [RRR], Cleveland National Forest [CNF], Vista Irrigation District [VID]), and one at VID Lake Henshaw (VLH) that has been surveyed annually since 2018. There were 71 territorial flycatchers detected at 3 locations (RRR, CNF, VLH), and 6 transient flycatchers of unknown subspecies detected at VID and VLH. Downstream from Lake Henshaw, four territorial flycatchers, including two males and two females, were detected at RRR and CNF. In total, two territories were established consisting of two pairs at these locations. At VLH, we detected 67 territorial flycatchers, including 30 males, 34 females, and 3 flycatchers of unknown sex. In total, 40 territories were established, containing 35 pairs (24 monogamous pairings and 5 polygynous groups consisting of 4 males each pairing with 2 different females, and 1 male pairing with 3 different females), and 5 flycatchers of undetermined breeding status (3 males and 2 flycatchers of unknown sex). Brown-headed cowbirds (Molothrus ater; cowbird) were detected at all four survey locations.

Flycatchers used five habitat types in the survey area: (1) mixed willow riparian, (2) willow-cottonwood, (3) willow-oak, (4) willow-ash, and (5) oak-sycamore. Of the flycatcher locations, 83 percent were located in habitat characterized as mixed willow riparian, and 92 percent were in habitat with greater than 95-percent native plant cover. Exotic vegetation was not prevalent in the survey area.

There were 22 nests incidentally located during surveys: 5 were successful, 1 was seen with eggs on the last visit, 10 failed, and the outcome of the remaining 6 nests was unknown. Three of these nests were parasitized by cowbirds. There were 13 juveniles detected at VLH; no juveniles were detected at RRR or CNF.

Five banded flycatchers were detected during surveys, three of which were confirmed to be adults that held territories in previous years. In addition, two flycatchers with a single dark blue federal band, indicating that they were banded as nestlings in a previous demographic study downstream from Lake Henshaw (Howell and others, 2022), were resighted during surveys.

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 including southern California, Arizona, New Mexico, extreme southern parts of Nevada and Utah, and western Texas (Hubbard, 1987; Unitt, 1987). Restricted to riparian habitat for breeding, the flycatcher has declined within the past five decades in response to widespread habitat loss throughout its range and, possibly, brood-parasitism by the Brown-headed Cowbird (Molothrus ater; cowbird [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 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 effort 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).

Flycatchers in southern California co-occur with the Least Bell’s Vireo (Vireo bellii pusillus; vireo), another riparian obligate endangered by habitat loss and cowbird parasitism. Unlike the vireo, which has increased tenfold since the mid-1980s in response to management practices alleviating threats (U.S. Fish and Wildlife Service, 2006), the number of flycatchers has remained low. Currently (2016), the majority of flycatchers in California are concentrated at two known sites: (1) the upper San Luis Rey River at Lake Henshaw in San Diego County (Howell and Kus, 2022b) and (2) the Owens River Valley in Inyo County (Greene and others, 2021). Outside of these sites, flycatchers occur as small, isolated populations of five territories or less.

Male flycatchers begin arriving in southern California in early to mid-May, whereas females arrive approximately 1 week later. While on the breeding grounds, males sing repeatedly from exposed perches. Once the pair bond is established, the female builds an open cup nest that 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 and early September for their wintering grounds in Central America and northern South America.

Flycatcher breeding habitat is characterized by patches of dense riparian vegetation along rivers, streams, and reservoir inflows, interspersed with small openings, open water, or areas of sparse vegetation. Vegetation species composition varies across the range, but most breeding habitats include tree or shrub cover that is at least 3 m tall, with patches of dense vegetation within 3–4 m of the ground. In addition, flycatcher breeding habitat is almost always near or adjacent to areas of standing water or saturated soil (U.S. Fish and Wildlife Service, 2002; Sogge and others, 2010).

The goal of the 2022 effort was to assess the population status, banding status, breeding status, and habitat attributes of the flycatcher population along the upper San Luis Rey River, in an area downstream from Lake Henshaw, where demographic monitoring occurred from 2015 to 2019 (Howell and others, 2022; Howell and Kus, 2022b), and the habitat surrounding Lake Henshaw. This report is the annual update to surveys that have been completed since 2015 (Howell and Kus, 2021, 2022a, b). The data contained in this report can be found in the associated data release (Howell and Kus, 2022b).

These data, when compared with data from other sites, will inform natural resource managers about the status of the flycatcher on the upper San Luis Rey River and guide modification of land-use and management practices as appropriate to ensure the species’ continued existence.

Methods

Study Area

The study area consisted of an approximately 6.9-kilometer (km; 4.3-miles [mi]) reach of the upper San Luis Rey River downstream from Lake Henshaw and the habitat surrounding Lake Henshaw (fig. 1). Four locations along the upper San Luis Rey River were surveyed for flycatchers in 2022. Three locations were downstream from Lake Henshaw (Rey River Ranch [RRR], Cleveland National Forest [CNF], and Vista Irrigation District [VID]) and were previously surveyed in 2015–21. One location was upstream from the dam, VID Lake Henshaw (VLH), and was previously surveyed in 2018–21. The study area included property managed by Vista Irrigation District, Cleveland National Forest, and private and county property downstream from the Forest Service property.

1. Overview of the study area with colored lines for described features.
Figure 1.

Location of Southwestern Willow Flycatcher (Empidonax traillii extimus) survey area on the upper San Luis Rey River, San Diego County, California, 2022.

Surface flows downstream were regulated by a dam at Lake Henshaw operated by the Vista Irrigation District and water was present year-round. In most years, spring and summer flows were swift, and slow-moving backwater/marshy habitats were absent. In 2022, however, there was very little water in the San Luis Rey River, and some sections were completely dry. The floodplain in the downstream part of the study area was narrow and bordered by steep slopes that supported chaparral vegetation. Riparian habitat downstream included a diverse mix of mature willow (Salix spp.) woodland and coast live oak (Quercus agrifolia) woodland, dominated by coast live oak, willow, velvet ash (Fraxinus velutina), California sycamore (Platanus racemosa), and white alder (Alnus rhombifolia). Thick understory vegetation was present, including wild rose (Rosa californica), poison oak (Toxicodendron diversilobum), stinging nettle (Urtica dioica), and California blackberry (Rubus ursinus), interspersed with patches of open habitat dominated by annual grasses and bracken fern (Pteridium sp.). The habitat surrounding Lake Henshaw was dominated by Goodding’s black willow (Salix gooddingii), with some arroyo willow (Salix lasiolepis), red willow (Salix laevigata), Fremont cottonwood (Populus fremontii), and coast live oak where the west fork of the San Luis Rey River and several other minor creeks flowed into the lake. There were several patches of non-native tamarisk (Tamarix ramosissima) further from the shoreline of the lake.

Surveys

U.S. Geological Survey (USGS) biologists Scarlett Howell, Suellen Lynn, and Devin Taylor completed flycatcher surveys following standard survey techniques for flycatchers (Sogge and others, 2010). Flycatcher surveys were completed under U.S. Fish and Wildlife Service (USFWS) 10(a)1(A) Recovery Permit ESPER0004080_0.1. Four surveys were performed at least 5 days apart during three consecutive survey periods between May 20 and July 28, 2022. Surveys were completed between dawn and early afternoon, avoiding periods of inclement weather such as temperatures below freezing, rain, or strong winds that inhibit detection of flycatchers. Surveys were done by walking next to the river or lake, using caution to avoid disturbing the habitat or damaging nests. In wider stands, observers traversed the habitat, choosing routes that permitted detection of all birds throughout its extent, such as multiple straight transects, serpentine, zig-zag, or criss-cross routes.

Upon initiation of the survey, investigators stood quietly for 1–2 minutes, listening for spontaneously singing flycatchers and acclimating to surrounding conditions, such as road and river noise. If there were no birds detected during the initial listening period, investigators broadcasted the flycatcher song (fitz-bew) using an MP3 player or Android phone and an amplified speaker at the volume of typical bird songs for approximately 10–15 seconds and then looked and listened for approximately 1 minute for a response. Song playback was ceased immediately upon detection of a flycatcher. Flycatchers typically responded by moving silently toward the song, singing in response to the song or producing some other call or vocalization. This procedure was repeated (including 10-second quiet pre-broadcast listening period) every 20–30 m throughout the survey site and more often if background noise was loud. If a flycatcher was detected, the investigator moved approximately 50–80 m beyond the detection before additional playback occurred to avoid double counting birds.

For each flycatcher encountered, observers recorded age (adult or juvenile), sex (male, female, or unknown), breeding status (paired, undetermined, or transient), and whether the bird was banded. A flycatcher was considered transient if detected only once, or if more than once, detections were less than 2 weeks apart. The flycatcher locations were mapped using Environmental Systems Research Institute (Esri) Field Maps (Environmental Systems Research Institute, 2022) on an Android phone with 1- to 15-m accuracy to determine geographic coordinates (World Geodetic System of 1984 [WGS 84]). Dominant native and exotic plants were recorded at each location, and percent cover of native vegetation was estimated using cover categories of less than 5 percent, 5–50 percent, 51–95 percent, and greater than 95 percent. Overall habitat type was specified according to the following categories:

  • Mixed willow riparian: Habitat dominated by one or more willow species, including Goodding’s black willow, arroyo willow, red willow, and sandbar willow (Salix exigua), with mule fat (Baccharis salicifolia) as a frequent co-dominant.

  • Willow-ash: Willow riparian habitat in which velvet ash is a co-dominant.

  • Willow-cottonwood: Willow riparian habitat in which Fremont cottonwood is a co-dominant.

  • Willow-oak: Willow-riparian habitat in which coast live oak is a co-dominant.

  • Willow-sycamore: Willow riparian habitat in which California sycamore is a co-dominant.

  • Oak-sycamore: Woodlands in which coast live oak and California sycamore occur as co-dominants.

  • Non-native: Areas vegetated exclusively with non-native species, such as giant reed (Arundo donax) and tamarisk.

Breeding Activities

We documented any evidence of breeding (for example, a female with nest material or a completed nest, adults carrying food, or dependent juveniles in the territory) observed during surveys. Incidental nest locations observed during surveys were recorded and the contents observed whenever possible.

Brown-headed Cowbirds

We documented cowbird presence during surveys. Whenever possible, the contents of incidentally located flycatcher nests were observed for cowbird eggs. If present, cowbird eggs were removed from the nest and destroyed to promote nest success because parasitized flycatcher nests are rarely successful in fledging host young (Rothstein and others, 2003).

Banding

Flycatchers were banded at three locations (RRR, CNF, and VID) as part of a separate demographic study from 2015 to 2019 (Howell and others, 2022). In that study, adults were captured at monitored territories using mist nets and song playback and were banded with a unique color-band combination. Nestlings from accessible nests were banded with a single metal dark blue band on the left or right leg. In subsequent years, flycatchers that were resighted with a single dark blue band (natal) were recaptured using the same methods described for adults and given a second leg band to yield a unique color-band combination. In 2022, we attempted to resight all flycatchers to identify individuals based on color-band combinations. Color-band resighting data were used to determine age and document movement from banding sites.

Results

Distribution and Abundance

In 2022, there were 71 territorial flycatchers and 6 transient flycatchers of unknown subspecies observed at the 4 survey locations along the upper San Luis Rey River (fig. 2; tables 1, 2). Of the 71 territorial flycatchers, 32 were males, 36 were females, and 3 were of unknown sex. The flycatcher population at the upper San Luis Rey River decreased by 9 percent from 2021 (78 territorial flycatchers; Howell and Kus, 2022a, b) to 2022 (Howell and Kus, 2022b).

2. Overview of the study area with colored lines and symbols for described features.
Figure 2.

Southwestern Willow Flycatcher (Empidonax traillii extimus) detections and breeding status on the upper San Luis Rey River, San Diego County, California, 2022.

Table 1.    

Total number and breeding status of Willow Flycatchers (Empidonax traillii) detected in the study area on the upper San Luis Rey River, San Diego County, California, 2022.

[Survey location: RRR, Rey River Ranch; CNF, Cleveland National Forest; VID, Vista Irrigation District; VLH, VID Lake Henshaw. Abbreviations: Juv., juveniles; Unk., unknown]
Survey
location		 Number of Breeding status
Transient
flycatchers		 Territorial
flycatchers		 Males Females Unk.
sex		 Juv. Territories Pair(s) Undetermined
RRR 0 2 1 1 0 0 1 1 0
CNF 0 2 1 1 0 0 1 1 0
VID 5 0 0 0 0 0 0 0 0
VLH 1 67 30 34 3 13 40 35 5
Total 6 71 32 36 3 13 42 37 5
Table 1.    Total number and breeding status of Willow Flycatchers (Empidonax traillii) detected in the study area on the upper San Luis Rey River, San Diego County, California, 2022.

Table 2.    

Locations, breeding status, and band status of Willow Flycatchers (Empidonax traillii) detected in the study area on the upper San Luis Rey River, San Diego County, California, 2022.

[Survey location: CNF, Cleveland National Forest; RRR, Rey River Ranch; VID, Vista Irrigation District; VLH, VID Lake Henshaw. Breeding Status: P, pair; T, transient (subspecies unknown); U, undetermined. Sex: F, female; M, male. Banded Bird (s) present: Y, yes; U, unknown; N, no. Other abbreviations: ID, identification; &, and; —, no additional comment]
Survey
location		 Bird
ID		 Number
of adults		 Breeding
status		 Sex Banded
bird(s) present		 Comments
CNF CNF01F 2 P M & F Y Male banded.
RRR RRR01F 2 P M & F U Did not resight female.
VID VID01F 1 T U N —
VID VID02F 1 T U U —
VID VID03F 1 T U N —
VID VID04Fa 1 T U N —
VID VID04Fb 1 T U N —
VLH LHW01F 2 P M & F Y Male banded.
VLH LHW02F 2 P M & F N Polygynous male (LHW02/04F).
VLH LHW03F 2 P M & F N —
VLH LHW04F 1 P F Y Female banded. Second female of LHW02F.
VLH LHW05F 2 P M & F N —
VLH LHW06F 2 P M & F N —
VLH LHW07F 2 P M & F U Did not resight male.
VLH LHW08F 2 P M & F N —
VLH LHW09F 2 P M & F U Did not resight male.
VLH LHW10F 2 P M & F N —
VLH LHW11F 2 P M & F N —
VLH LHW12F 2 P M & F N —
VLH LHW13F 2 P M & F N Polygynous male (LHW13/23F).
VLH LHW14F 2 P M & F Y —
VLH LHW15F 2 P M & F N —
VLH LHW16F 1 T M N —
VLH LHW17F 2 P M & F N —
VLH LHW19F 2 P M & F N Polygynous male (LHW19/27F).
VLH LHW21F 2 P M & F N —
VLH LHW23F 1 P F N Second female of LHW13F.
VLH LHW25F 2 P M & F Y Male banded.
VLH LHW27F 1 P F N Second female of LHW19F.
VLH LHW29F 1 U M N —
VLH MLH01F 2 P M & F U Did not resight female.
VLH MLH02F 1 U M N —
VLH VLH01F 2 P M & F U Did not resight female.
VLH VLH02F 2 P M & F N —
VLH VLH03F 2 P M & F N —
VLH VLH04F 2 P M & F N Polygynous male (VLH04/13/16F).
VLH VLH05F 2 P M & F N —
VLH VLH06F 1 U U N —
VLH VLH07F 2 P M & F N —
VLH VLH08F 2 P M & F N Polygynous male (VLH8/11F).
VLH VLH09F 2 P M & F N —
VLH VLH10F 1 U M N —
VLH VLH11F 1 P F U Did not resight female. Second female of VLH08F.
VLH VLH12F 1 P U N Likely second female of VLH09F.
VLH VLH13F 1 P F N Third female of VLH04F.
VLH VLH14F 1 U U N —
VLH VLH15F 1 P F N Likely second female of VLH05F.
VLH VLH16F 1 P F N Second female of VLH04F.
Table 2.    Locations, breeding status, and band status of Willow Flycatchers (Empidonax traillii) detected in the study area on the upper San Luis Rey River, San Diego County, California, 2022.

A total of four territorial flycatchers (two males and two females) were detected at CNF and RRR (fig. 2; tables 1, 2). In total, two territories consisting of two pairs were established. No territorial flycatchers were observed at VID, but five transient individuals were detected. The number of territorial flycatchers observed downstream from Lake Henshaw decreased by 20 percent compared to 2021 (5 territorial flycatchers; Howell and Kus, 2022a, b).

A total of 67 territorial flycatchers were detected at VLH (fig. 2; tables 1, 2), including 30 males, 34 females, and 3 flycatchers of unknown sex. There were 40 territories established, containing 35 pairs (24 monogamous pairings and 5 polygynous groups consisting of 4 males each pairing with 2 different females, and 1 male pairing with 3 different females) and 5 flycatchers of undetermined breeding status (3 males and 2 flycatchers of unknown sex). The number of territorial flycatchers observed at VLH decreased by 8 percent compared to 2021 (73 territorial flycatchers; Howell and Kus, 2022a, b). In addition, one transient flycatcher was detected (fig. 2; tables 1, 2).

The distribution of flycatcher territories along the upper San Luis Rey River was similar relative to 2021. In 2022, 95 percent of all territories (40/42) were at VLH, and 5 percent (2/42) were downstream from Lake Henshaw, compared to 2021 when 93 percent of all territories (43/46) were at VLH and 7 percent (3/46) downstream. From 2018 to 2022, the combined population of flycatchers downstream from Lake Henshaw decreased annually, whereas the population at VLH increased from 2018 to 2021 (fig. 3). For the first time since 2018, we observed a slight decrease in the number of flycatcher territories at VLH from 2021 to 2022 (fig. 3).

3. Flycatcher territories downstream from Lake Henshaw declined annually from 2018 to 2022, increased upstream from 2018 to 2021, and decreased upstream from 2021 to 2022.
Figure 3.

Distribution of Southwestern Willow Flycatcher (Empidonax traillii extimus) territories on the upper San Luis Rey River, San Diego County, California, 2018–22. [CNF, Cleveland National Forest; RRR, Rey River Ranch; VID, Vista Irrigation District; VLH, VID Lake Henshaw]

Flycatchers used five habitat types in the survey area. Of the flycatcher locations, 83 percent (40/48) occurred in habitat characterized as mixed willow riparian, 13 percent (6/48) in willow riparian habitats co-dominated by cottonwood, oak, or ash, and 4 percent (2/48) occurred in oak-sycamore. The most frequently recorded species at flycatcher locations included Goodding’s black willow, coast live oak, and Fremont cottonwood. Exotic vegetation was not prevalent in the survey area; 92 percent (44/48) of flycatcher locations occurred in habitat with greater than 95-percent native plant cover (table 3).

Table 3.    

Habitat characteristics of Willow Flycatcher (Empidonax trailli) locations on the upper San Luis Rey River, San Diego County, California, 2022.

[Survey location: CNF, Cleveland National Forest; RRR, Rey River Ranch; VID, Vista Irrigation District; VLH, VID Lake Henshaw. Mixed willow riparian: Habitat dominated by one or more willow species, including Goodding’s black willow, arroyo willow, red willow, and sandbar willow, with mule fat as frequent co-dominant. Oak-sycamore: Woodlands in which coast live oak and California sycamore occur as co-dominants. Willow-ash: Willow riparian habitat in which velvet ash is a co-dominant. Willow-cottonwood: Willow riparian habitat in which Fremont cottonwood is a co-dominant. Willow-oak: Willow riparian habitat in which coast live oak is a co-dominant. Other abbreviations: ID, identification; >, greater than; —, no data]
Survey
location		 Bird
ID		 Habitat
type		 Dominant
species		 Percent
native cover		 Dominant
exotic
species
CNF CNF01F Willow-ash Velvet ash >95 —
RRR RRR01F Oak-sycamore Coast live oak 50–95 Black mustard (Brassica nigra)
VID VID01F Willow-oak Red or Arroyo willow >95 —
VID VID02F Willow-oak Coast live oak >95 —
VID VID03F Oak-sycamore Coast live oak >95 —
VID VID04Fa Mixed willow Sandbar willow >95 —
VID VID04Fb Mixed willow Sandbar willow >95 —
VLH LHW01F Mixed willow Goodding's black willow >95 —
VLH LHW02F Mixed willow Goodding's black willow >95 —
VLH LHW03F Mixed willow Goodding's black willow >95 —
VLH LHW04F Mixed willow Goodding's black willow >95 —
VLH LHW05F Mixed willow Goodding's black willow >95 —
VLH LHW06F Mixed willow Goodding's black willow >95 —
VLH LHW07F Mixed willow Goodding's black willow >95 —
VLH LHW08F Mixed willow Goodding's black willow >95 —
VLH LHW09F Mixed willow Goodding's black willow >95 —
VLH LHW10F Mixed willow Goodding's black willow >95 —
VLH LHW11F Mixed willow Goodding's black willow >95 —
VLH LHW12F Mixed willow Goodding's black willow >95 —
VLH LHW13F Mixed willow Goodding's black willow >95 —
VLH LHW14F Mixed willow Goodding's black willow >95 —
VLH LHW15F Mixed willow Goodding's black willow >95 —
VLH LHW16F Mixed willow Goodding's black willow >95 —
VLH LHW17F Mixed willow Goodding's black willow >95 —
VLH LHW19F Mixed willow Goodding's black willow >95 —
VLH LHW21F Mixed willow Goodding's black willow >95 —
VLH LHW23F Mixed willow Goodding's black willow >95 —
VLH LHW25F Mixed willow Goodding's black willow >95 —
VLH LHW27F Mixed willow Goodding's black willow >95 —
VLH LHW29F Mixed willow Goodding's black willow >95 —
VLH MLH01F Mixed willow Goodding's black willow >95 —
VLH MLH02F Mixed willow Goodding's black willow 50–95 Tamarisk
VLH VLH01F Mixed willow Goodding's black willow >95 —
VLH VLH02F Mixed willow Goodding's black willow >95 —
VLH VLH03F Mixed willow Goodding's black willow >95 —
VLH VLH04F Mixed willow Goodding's black willow >95 —
VLH VLH05F Mixed willow Goodding's black willow >95 —
VLH VLH06F Mixed willow Goodding's black willow >95 —
VLH VLH07F Willow-cottonwood Fremont cottonwood >95 —
VLH VLH08F Mixed willow Goodding's black willow >95 —
VLH VLH09F Mixed willow Goodding's black willow >95 —
VLH VLH10F Willow-cottonwood Fremont cottonwood >95 —
VLH VLH11F Mixed willow Goodding's black willow >95 —
VLH VLH12F Mixed willow Goodding's black willow 50–95 White sweetclover (Melilotus albus)
VLH VLH13F Mixed willow Goodding's black willow >95 —
VLH VLH14F Mixed willow Goodding's black willow 50–95 White sweetclover (Melilotus albus)
VLH VLH15F Willow-cottonwood Fremont cottonwood >95 —
VLH VLH16F Mixed willow Goodding's black willow >95 —
Table 3.    Habitat characteristics of Willow Flycatcher (Empidonax trailli) locations on the upper San Luis Rey River, San Diego County, California, 2022.

Breeding Activities

Nest building was observed during the first survey period in May, and 22 flycatcher nests were incidentally located during surveys. Of the 22 nests, 5 successfully fledged at least one flycatcher young, 1 nest was seen with eggs on the last survey, 10 nests failed, and the outcome of the remaining 6 nests was unknown. A minimum of 13 flycatcher juveniles were seen at VLH during the study period (table 1), including 8 from incidentally found nests and 5 detected during surveys. No juveniles were observed at RRR or CNF.

Brown-headed Cowbirds

Cowbirds were detected at all four survey locations. Three flycatcher nests were observed with cowbird eggs: two nests each contained one cowbird egg, and one nest contained two cowbird eggs. The cowbird egg was removed from one of the parasitized nests, but the nesting attempt failed despite removal. The other two parasitized nests also failed. Flycatchers in two additional territories at VLH were each observed feeding a cowbird fledgling.

Banded Birds

Five banded flycatchers, all banded prior to 2022, were detected on the upper San Luis Rey River in 2022 (tables 4, 5). Banded flycatcher ages ranged from 4 to 7 years old (tables 4, 5).

Three banded flycatchers (two males and one female) with unique color-band combinations were resighted in 2022; all were previously detected as adults on the upper San Luis Rey River in 2021 (table 4). Of the three color-banded flycatchers, one male originally banded as an adult in 2018 was detected in CNF. The remaining two uniquely color-banded adults (one male and one female) were detected at VLH. The female was originally banded as an adult at CNF in 2016, and the male was originally banded as a nestling at CNF in 2017 and recaptured and given a unique color-band combination at RRR in 2018; both moved to VLH in 2021. In 2022, all banded adult flycatchers returned to the same location they occupied in 2021.

Table 4.    

Band status and movement of adult Southwestern Willow Flycatchers (Empidonax traillii extimus) detected on the upper San Luis Rey River, San Diego County, California, 2022.

[2022 Location/territory: CNF, Cleveland National Forest; VLH, VID Lake Henshaw. Sex: F, female; M, male. Age originally banded: A, adult; N, nestling. Year/location originally banded/previously seen: CNF, Cleveland National Forest; VLH, VID Lake Henshaw. Other abbreviations: km, kilometer]
2022
Location/
territory		 Sex Minimum
age in 2022
(years)		 Age
originally
banded		 Year/location
originally
banded		 Year/location
previously
seen		 Distance
moved
(km)
CNF/CNF01F M 5 A 2018/CNF 2021/CNF 0
VLH/LHW04F F 7 A 2016/CNF 2021/VLH 0
VLH/LHW14F M 5 N 2017/CNF 2021/VLH 0
Table 4.    Band status and movement of adult Southwestern Willow Flycatchers (Empidonax traillii extimus) detected on the upper San Luis Rey River, San Diego County, California, 2022.

Table 5.    

Band status and movement of natal Southwestern Willow Flycatchers (Empidonax traillii extimus) detected on the upper San Luis Rey River, San Diego County, California, 2022.

[2022 Location/territory: VLH, VID Lake Henshaw. Sex: M, male. Distance moved (km): minimum and maximum values represent estimates for flycatchers whose exact natal locations were unknown; min, minimum distance calculated between the closest successful nest in the bird’s natal year to the first adult location; max, maximum distance calculated between the farthest successful nest in the bird’s natal year to the first adult location. Other abbreviations: km, kilometer; —, no data]
2022
Location/
territory		 Sex Minimum
age in 2022
(years)		 Year
originally
banded		 Distance moved
(km)
Minimum Maximum
VLH/LHW25F M 4 2018 — 6.8
VLH/LHW01F M 6 2016 or 2018 2.3 6.7
Table 5.    Band status and movement of natal Southwestern Willow Flycatchers (Empidonax traillii extimus) detected on the upper San Luis Rey River, San Diego County, California, 2022.

Two natal flycatchers (two males) banded as nestlings were detected on the upper San Luis Rey River in 2022 at VLH (table 5). Of the two natal birds, one was originally banded in 2016 and the second in 2018 (table 5). The dispersal distances moved by natal flycatchers from the former demographic study area to VLH were estimated to range from 2.3 km (minimum) to 6.8 km (maximum).

Summary

In 2022, the overall population of Southwestern Willow Flycatchers on the upper San Luis Rey River near Lake Henshaw declined slightly compared to 2021 (9 percent; from 78 territorial flycatchers to 71 territorial flycatchers). The distribution of birds in the study area was similar to that seen in 2021, with the majority of territories at Lake Henshaw. The number of territories downstream from Lake Henshaw decreased by 33 percent from 2021 (3) to 2022 (2), whereas the number of territories observed at Lake Henshaw decreased by 7 percent from 2021 (43) to 2022 (40). This is the first year since the Lake Henshaw population was discovered that a decline was documented.

The Southwestern Willow Flycatcher population in California appears to be experiencing a statewide decline that is not isolated to the upper San Luis Rey River near Lake Henshaw. Populations on the lower San Luis Rey River (Houston and others, 2021), the Santa Margarita River on Marine Corps Base Camp Pendleton (B.E. Kus, U.S. Geological Survey, unpub. data, 2022), and the Kern River (M.J. Whitfield, Southern Sierra Research Station, written commun., 2020) have steeply declined or have been extirpated in recent years. As of 2022, the population along the upper San Luis Rey River near Lake Henshaw is the largest recorded Southwestern Willow Flycatcher population in southern California, making it central to understanding the conditions that favor and promote flycatchers and their habitat.

References Cited

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Environmental Systems Research Institute, 2022, Field Maps release 22.3.1: Redlands, Calif., Environmental Systems Research Institute.

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Houston, A., Allen, L.D., Pottinger, R.E., and Kus, B.E., 2021, Least Bell's Vireos and Southwestern Willow Flycatchers at the San Luis Rey flood risk management project area in San Diego County, California—Breeding activities and habitat use—2020 annual report: U.S. Geological Survey Open-File Report 2021–1053, 67 p., accessed October 21, 2021, at https://doi.org/10.3133/ofr20211053.

Howell, S.L., and Kus, B.E., 2021, Distribution and abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the upper San Luis Rey River, San Diego County, California—2020 data summary: U.S. Geological Survey Data Series 1140, 11 p., accessed October 4, 2021, at https://doi.org/10.3133/ds1140.

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Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain
Length
meter (m) 3.281 foot (ft)
kilometer (km) 0.6214 mile (mi)

Datums

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

Abbreviations

CNF

Cleveland National Forest

RRR

Rey River Ranch

VID

Vista Irrigation District

VLH

VID Lake Henshaw

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

Suggested Citation

Howell, S.L., and Kus, B.E., 2023, Distribution and abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the upper San Luis Rey River, San Diego County, California—2022 data summary: U.S. Geological Survey Data Report 1173, 12 p., https://doi.org/10.3133/dr1173.

ISSN: 2771-9448 (online)

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Distribution and abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the upper San Luis Rey River, San Diego County, California—2022 data summary
Series title Data Report
Series number 1173
DOI 10.3133/dr1173
Year Published 2023
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description Report: vi, 12 p.; Data Release
Country United States
State California
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details