Distribution and Abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Upper San Luis Rey River, San Diego County, California—2021 Data Summary
In 2021, researchers surveyed Southwestern Willow Flycatchers (Empidonax traillii extimus) along the upper San Luis Rey River near Lake Henshaw in Santa Ysabel, California, at four locations: three downstream from Lake Henshaw (Rey River Ranch [RRR], Cleveland National Forest [CNF], Vista Irrigation District [VID]) and one at VID Lake Henshaw (VLH). They detected 78 territorial flycatchers at three sites (RRR, CNF, VLH) and one transient flycatcher of unknown subspecies at VID. Downstream sites accounted for five flycatchers forming three territories, while VLH supported 73 flycatchers forming 43 territories, including 38 pairs, several polygynous groupings, and five individuals of undetermined breeding status. Brown-headed cowbirds (Molothrus ater) were recorded at all sites. Flycatchers primarily used five habitat types—mixed willow riparian, willow-cottonwood, willow-oak, willow-ash, and sycamore-oak—with the majority found in mixed willow riparian with high native plant cover. Fifteen nests were incidentally located, of which one was successful, two contained eggs or nestlings, nine failed, and three had unknown outcomes; three nests experienced cowbird parasitism. Thirteen juveniles were detected at VLH, and resightings of 10 banded flycatchers confirmed that seven were returning adults, while three previously banded as nestlings downstream were also observed. The study documented dispersal from downstream sites to Lake Henshaw, with three natal and three adult flycatchers moving upstream in 2021.
Howell, S.L., and Kus, B.E., 2022, Distribution and abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Upper San Luis Rey River, San Diego County, California—2021 data summary: Data Report 1158, 11p., https://doi.org/10.3133/dr1158.