Study Area

The study area consisted of an approximately 6.9-kilometer (km) 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 2023. Three locations were downstream from Lake Henshaw (Rey River Ranch [RRR], Cleveland National Forest [CNF], and Vista Irrigation District [VID]) and were previously surveyed annually from 2015 to 2022. One location was upstream from the dam, VID Lake Henshaw (VLH), and was previously surveyed annually from 2018 to 2022. The study area included property managed by Vista Irrigation District, Cleveland National Forest, and private 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. In most years, spring and summer flows were swift and slow-moving backwater/marshy habitats were absent. In 2023, however, there was very little water in the downstream part of the San Luis Rey River, and some sections were dry by the end of July because of a reduced release schedule coordinated among the Vista Irrigation District, the City of Escondido, and tribal representatives during treatment for a harmful algal bloom at the lake (D. Smith, Vista Irrigation District, written commun., 2023). 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 California wildrose (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) present. In 2023, above average precipitation (114.5 centimeters [cm], 172 percent of mean from 1960 to 2022 [66.7 cm]; J. Sherwood, Vista Irrigation District, written commun., 2023) partially inundated flycatcher habitat at Lake Henshaw. All understory vegetation was submerged but the tops of mature willow, cottonwood, oak, and tamarisk trees extended above the water line in some sections.

Surveys

U.S. Geological Survey biologists Lisa Allen, Scarlett Howell, Barbara Kus, Megan Logsdon, Suellen Lynn, Shannon Mendia, and Ryan Pottinger completed flycatcher surveys following a standardized call back survey protocol for Southwestern Willow Flycatchers (Sogge and others, 2010). The survey protocol is designed to increase the likelihood of detecting Willow Flycatchers and aid in determining their breeding status by performing repeated surveys during the early to mid-nesting season, with four surveys carried out at least 5 days apart during three consecutive survey periods between May 15 and July 31. One survey was carried out between May 15 and May 31, one survey between June 1 and June 24, and two surveys between June 25 and July 31. Flycatcher surveys were completed under U.S. Fish and Wildlife Service (USFWS) 10(a)1(A) Recovery Permit ESPER0004080_0.2. 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. In flooded habitat, surveys were done by boat, primarily by moving along the outside edge of the habitat patch and entering the interior habitat whenever possible. Because we were unable to access the interior habitat in many sections at Lake Henshaw, the number of flycatcher detections in 2023 should be considered a minimum.

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. During boat surveys, investigators stopped the boat engine and floated quietly during the listening period. If there were no birds detected during the initial listening period, investigators broadcasted 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. Additional flycatcher vocalizations were broadcasted occasionally during boat-based surveys to elicit response and confirm flycatcher presence. This procedure was repeated (including a 10-second quiet pre-broadcast listening period) every 20–30 m throughout land-based survey sites or 80–100 m throughout boat-based survey sites, and more frequently 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. At most flycatcher territories, flycatchers in adjacent territories could be heard vocalizing simultaneously.

For each flycatcher encountered, observers recorded age (adult or juvenile), sex (male, female, or unknown), breeding status (paired, undetermined, or transient), and whenever possible, whether the bird was banded. Flycatchers were considered paired if a second flycatcher was present and unchallenged by the territorial male during more than one survey period, if male/female interactions were observed, if an adult flycatcher was observed carrying nesting material or food, if an active flycatcher nest was located, or if adults were observed actively feeding fledglings. To avoid overcounting male flycatchers, observers also attempted to determine pairing type (monogamous or polygynous). Monogamous pairings consisted of one male paired with one female, whereas polygynous pairings consisted of one male paired with more than one female. Behaviors used to establish polygyny included males interacting with more than one female simultaneously or sequentially. For example, during a territory visit observers often documented a male/female interaction in territory “A” while simultaneously hearing a third flycatcher vocalizing (for example, whitting) in the adjacent territory “B,” and when the territory A interaction ended, a male/female interaction was subsequently heard in territory B, and the female in territory A simultaneously began vocalizing. Although we were able to determine pairing type in most territories, occasionally we were unable to determine which adjacent male a female was paired with, especially in locations where multiple territories were clustered together. For example, a female in territory B was known to be paired because we documented male/female interactions at her location, but she could be paired with the territory A male directly south who was initially thought to be monogamous, or with the territory “C” male directly north who was already known to be polygynous with two different females. Flooded conditions that prevented us from accessing interior habitat further complicated the determination of pairing type in 2023, because we were often unable to use banded bird resights, nest locations, or other territory observations to determine which male (A or C) was paired with female B. For this reason, we present a range for the number of monogamous and polygynous pairs. Flycatcher breeding status was considered undetermined when behaviors such as spontaneous singing or other territory defense were observed during the non-migrant period (approximately June 15 to July 20; Sogge and others, 2010) but no pair behaviors were confirmed. A flycatcher was considered transient if detected only once, or if more than once, detections were less than 2 weeks apart. Flycatcher locations were mapped using Environmental Systems Research Institute 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). 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:

Incidental Nesting Activities

We documented any evidence of nesting (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).

Banded Bird Resighting

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 2023, 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.

Abundance and Distribution

In 2023, there were a minimum of 74 territorial flycatchers and 12 transient flycatchers of unknown subspecies observed along the upper San Luis Rey River (fig. 2; tables 1, 2). Of the 74 territorial flycatchers, 31 were male, 40 were female, and 3 were of unknown sex. The flycatcher population at the upper San Luis Rey River increased by 4 percent from 2022 (71 territorial flycatchers; Howell and Kus, 2022b, 2023) to 2023 (Howell and Kus, 2022b). For the first time since annual surveys began in 2015, no territorial flycatchers were detected at any of the survey locations downstream from Lake Henshaw; all territorial detections were at VLH. There were 51 territories established at VLH, containing 40 pairs (23 males and 40 females) and 11 flycatchers of undetermined breeding status (8 males and 3 flycatchers of unknown sex). Of the 40 pairs, 9–11 were monogamous pairings, and 29–31 were polygynous pairings consisting of 9–11 males each pairing with 2 different females, and 1–3 males each pairing with 3 different females. Of the 12 transient flycatchers, 11 were detected at VLH, and 1 was detected at CNF (fig. 2; tables 1, 2).

The distribution of flycatcher territories along the upper San Luis Rey River has shifted since 2018 when Lake Henshaw was first surveyed. From 2018 to 2023, the combined population of flycatchers downstream from Lake Henshaw decreased annually before falling to zero territories in 2023, whereas the population upstream from the dam at VLH increased from 2018 to 2021, declined slightly in 2022, and increased again in 2023 (fig. 3).

Flycatchers used three habitat types in the survey area. Of the flycatcher locations, 86 percent (54/63) occurred in habitat characterized as mixed willow riparian, 10 percent (6/63) in willow riparian habitat co-dominated by cottonwood and the remainder occurred in oak-sycamore (table 3). The most frequently recorded dominant vegetation species at flycatcher locations was Goodding’s black willow. Exotic vegetation was not prevalent in the survey area; 95 percent (60/63) of flycatcher locations occurred in habitat with greater than 95-percent native plant cover (table 3).

Incidental Nesting Activities

Observers incidentally located five flycatcher nests during surveys at VLH. Of the five nests, one nest failed, one nest was seen with eggs on the last survey, and the outcome of the remaining three nests was unknown. No flycatcher juveniles were observed at any of the survey locations in 2023.

Brown-headed Cowbirds

Cowbirds were detected at all four survey locations. One of the five incidentally located flycatcher nests contained one cowbird egg. We also suspected that one nest contained a cowbird nestling but could not confirm. Flycatchers in two additional territories at VLH were each observed feeding a cowbird fledgling.

Banded Birds

None of the five banded flycatchers at VLH in 2022, or any banded flycatchers present in previous years were observed in 2023. However, flooded conditions made entering the interior habitat difficult, and we were unable to access the exact territory locations where banded birds were observed in 2022.