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 2021. 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 2015–20. One location was upstream from the dam, VID Lake Henshaw (VLH), and was previously surveyed in 2018–20. The study area included property managed by Vista Irrigation District, Cleveland National Forest, and private and county property downstream from the Forest Service property. Surface flows downstream were regulated by a dam at Lake Henshaw operated by the Vista Irrigation District and water was present year-round. Spring and summer flows were swift and slow-moving backwater/marshy habitats were absent. The flood plain 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 the lake was dominated by Gooding’s black willow (Salix goodingii), with some arroyo willow (Salix lasiolepis), red willow (Salix laevigata), Fremont cottonwood (Populus fremontii), and coast live oak present 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 outside edge of the lake.

Surveys

U.S. Geological Survey (USGS) biologists Scarlett Howell and Suellen Lynn completed flycatcher surveys following standard survey techniques for flycatchers (Sogge and others, 2010). Flycatcher surveys were performed under U.S. Fish and Wildlife Service (USFWS) 10(a)1(A) Recovery Permit ESPER0004080_0. Four surveys were done at least 5 days apart during three consecutive survey periods between May 15 and July 31, 2021, except for the CNF location, which was only surveyed three times because of access restrictions. Surveys were completed between dawn and early afternoon, depending on wind and weather conditions. Surveys were not done during inclement weather, such as temperatures below freezing, rain, or strong winds that inhibit detection of vocalizations. Surveys were performed by walking next to the river, 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 (min), 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 and an amplified speaker at the volume of normal 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 ESRI Collector (Environmental Systems Research Institute, 2020) 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:

Breeding Activities

We documented any breeding activities observed during surveys. Incidental nest locations observed during surveys were recorded and the contents observed whenever possible.

Brown-headed Cowbirds

We documented the presence of cowbirds during surveys. Whenever possible, the contents of incidentally located flycatcher nests were observed for the presence of 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 that was done from 2015 to 2019 (B.E. Kus, U.S. Geological Survey, unpub. data, 2015–19). 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, in monitored territories, 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 band combination. In 2021, 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.

Distribution and Abundance

In 2021, there were 78 territorial flycatchers and 1 transient flycatcher of unknown subspecies observed at the 4 survey locations along the upper San Luis Rey River (fig. 2; tables 1, 2). Of the 78 territorial flycatchers, 35 were males, 40 were females, and 3 were of unknown sex. The flycatcher population at the upper San Luis Rey River increased by 26 percent from 2020 (62 territorial flycatchers; Howell and Kus, 2021; Howell and Kus, 2022) to 2021.

A total of five flycatchers (three males and two females) were detected at CNF and RRR (fig. 2; tables 1, 2). No territorial flycatchers were observed at VID, but one transient individual was detected. In total, three territories were established, consisting of two pairs and one male of undetermined breeding status. The number of territorial flycatchers observed downstream from Lake Henshaw decreased by 55 percent compared to 2020 (11; Howell and Kus, 2021; Howell and Kus, 2022).

A total of 73 flycatchers were detected at VLH (fig. 2; tables 1, 2), including 32 males, 38 females, and 3 flycatchers of unknown sex. There were 43 territories established, containing 38 pairs (22 monogamous pairings; 7 confirmed polygynous pairings, consisting of 1 male and 2 females; and 1 suspected polygynous pairing, consisting of 1 male and 2 females) and 5 flycatchers of undetermined breeding status (2 males and 3 flycatchers of unknown sex). The number of territorial flycatchers observed at VLH increased by 43 percent compared to 2020 (51; Howell and Kus, 2021; Howell and Kus, 2022). Some of the increase at VLH can be attributed to increased access in 2021 because we were able to survey habitat that was flooded and inaccessible by foot in 2020.

The distribution of flycatcher territories along the upper San Luis Rey River changed relative to 2020. In 2021, 93 percent of all territories (43/46) were located at VLH and 7 percent (3/46) were downstream from Lake Henshaw, compared to 2020 when 80 percent of all territories (28/35) were located at VLH and 20 percent (7/35) downstream.

Flycatchers used five different habitat types in the survey area. Eighty-seven percent (41/47) of flycatchers were detected in habitat characterized as mixed willow riparian, 11 percent (5/47) were detected in willow riparian habitats co-dominated by cottonwood, oak, or ash, and 2 percent (1/47) were detected in sycamore-oak. Ninety-four percent (44/47) of flycatchers were detected in habitat with greater than 95-percent native plant cover (table 3).

The most commonly recorded dominant species at flycatcher locations included Gooding’s black willow and coast live oak. The most prevalent exotic species were tamarisk and black mustard (Brassica nigra).

Breeding Activities

There were 15 flycatcher nests incidentally located during surveys. Nest building was observed during the first survey period. Of the 15 nests, 1 successfully fledged at least one flycatcher young, 2 nests were seen with eggs or nestlings on the last survey, 9 nests failed, and the outcome of the remaining 3 nests was unknown. A minimum of 13 flycatcher juveniles were seen during the study period (table 1), including 2 from incidentally found nests and 11 detected during surveys. No juveniles were seen at RRR or CNF.

Brown-headed Cowbirds

Cowbirds were detected at all four survey locations. Three flycatcher nests were each observed to contain one cowbird egg. The cowbird eggs were removed from the nests, but all three nests failed despite removal. One flycatcher was observed feeding a cowbird fledgling.

Banded Birds

There were 10 banded flycatchers, all banded prior to 2021, that were detected on the upper San Luis Rey River in 2021 (tables 4, 5). Seven banded flycatchers (six males and one female) with unique color-band combinations were resighted in 2021; all were previously detected as adults on the upper San Luis Rey River in 2019 or 2020 (table 4). Of the seven color-banded flycatchers, three males were detected in the former demographic study area, two at RRR and one at CNF. Two of these males were originally banded as adults in 2016, and one male was originally banded as an adult in 2018. The remaining four full color-band combination adults (three males and one female) were detected at VLH. One male and one female were originally banded as adults at CNF in 2016. Two males were originally banded as nestlings but had been captured at CNF and given unique color-band combinations: one was originally banded as a nestling in 2017 and color banded in 2018, and one was originally banded as a nestling in 2018 and color banded in 2019.

In 2021, most of the adult flycatchers returned to the same location they last occupied; however, two males (last seen in 2019) and one female (last seen in 2020) moved from 3.2 to 7.3 km from the former demographic study area to VLH (table 4).

Three natal flycatchers (one male, one female, and one bird of unknown sex) were detected on the upper San Luis Rey River in 2021; all were detected at VLH (table 5). Of the three natal birds, one male and one bird of unknown sex were originally banded in 2016 or 2018 and the female was originally banded in 2017 (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 8.1 km (maximum). Banded flycatcher ages ranged from 3 to 6 years old (tables 4, 5).