Distribution and Abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Upper San Luis Rey River, San Diego County, California—2023 Data Summary
In 2023, researchers surveyed Southwestern Willow Flycatchers along the upper San Luis Rey River near Lake Henshaw, California, at four sites, including three downstream locations monitored since 2015 and one upstream site at Lake Henshaw surveyed annually since 2018. At the upstream site, they documented at least 74 territorial flycatchers across 51 territories, including 40 breeding pairs, with a mix of monogamous and polygynous pairings, while no territorial flycatchers were detected downstream for the first time since monitoring began. Most flycatchers were found in mixed willow riparian, willow–cottonwood, and oak–sycamore habitats with high native plant cover, and exotic vegetation was rare. Researchers located five nests, noting cowbird parasitism in several, and observed 12 transient flycatchers of unknown subspecies across sites, but no juveniles or banded individuals were recorded.
Howell, S.L., and Kus, B.E., 2024, Distribution and abundance of Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Upper San Luis Rey River, San Diego County, California—2023 data summary: U.S. Geological Survey Data Report 1194, 13 p., https://doi.org/10.3133/dr1194.