Distribution, Abundance, and Breeding Activities of Southwestern Willow Flycatchers (Empidonax traillii extimus) at Select Locations on the San Luis Rey River, San Diego County, California—2024 Data Summary
In 2024, researchers surveyed Southwestern Willow Flycatchers along the middle and upper San Luis Rey River in California, including six sites spanning Bonsall, Rincon, Rey River Ranch, Cleveland National Forest, Vista Irrigation District, and VID Lake Henshaw. At the upstream Lake Henshaw site, a minimum of 47 territorial flycatchers (26 males, 21 females) were detected, forming 30 territories with 21 pairs—13 monogamous and 8 polygynous—and 9 individuals of undetermined breeding status. No territorial flycatchers were observed downstream from Lake Henshaw or along the middle river sites, though six transient flycatchers of unknown subspecies were recorded across three locations. Flycatchers primarily used mixed willow riparian, willow-cottonwood, willow-oak, and willow-sycamore habitats, with 86 percent of detections in mixed willow riparian and 97 percent in areas with over 95 percent native plant cover. Nest monitoring at Lake Henshaw recorded 22 nests across 13 territories, of which nine were successful; failures were mostly due to predation or cowbird parasitism, with an overall parasitism rate of 16 percent. A total of 26 juvenile flycatchers were confirmed in 2024, providing key baseline data on nest success and productivity, while Brown-headed Cowbirds were detected at all survey locations and no banded flycatchers were observed.
Howell, S.L., and Kus, B.E., 2025, Distribution, abundance, and breeding activities of Southwestern Willow Flycatchers (Empidonax traillii extimus) at select locations on the San Luis Rey River, San Diego County, California—2024 data summary: U.S. Geological Survey Data Report 1212, 16 p., https://doi.org/10.3133/dr1212.