Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2023 Annual Report
In 2023, surveys for the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) were conducted across historically occupied riparian habitat at Marine Corps Base Camp Pendleton between May 8 and July 26. Fourteen transient Willow Flycatchers of unknown subspecies were observed at the Santa Margarita River and San Mateo Creek, while the resident population was limited to a single unpaired female in the Santa Margarita River Air Station breeding area. This female initiated one unsuccessful nesting attempt in late June, with no evidence of parasitism by Brown-headed Cowbirds. Notably, she was a three-year-old bird originally banded as a nestling on Base in 2020, marking the first return of a banded individual since 2012. Ongoing efforts to encourage breeding, including playback calls and artificial seeps, were in place near her territory, potentially contributing to her presence. Overall, the surveys highlighted low breeding activity, transient use of multiple habitats, and continued challenges to sustaining a resident flycatcher population on the Base.
Howell, S.L., and Kus, B.E., 2025, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2023 Annual report: U.S. Geological Survey Open-File Report 2025–1001, 33 p., https://doi.org/10.3133/ofr20251001.