The once common, endangered Southwestern Willow Flycatcher (Empidonax traillii extimus, SWFL) is rare throughout California, with scattered populations occurring at a few sites in the state. Most of these populations have declined over the past 10 to 20 years, however, the Owens River population has increased during this time. We monitored SWFLs on the Owens River during the 2021-2023 breeding seasons to determine abundance, nesting success, and nest parasitism rates. We estimated that there were at least 57 to 101 SWFL territories in our study area from 2021 to 2023, a significantly higher population than reported in 2016 (27). We recorded an average apparent nest success rate of 52% (n=38, 2021 and 2022); and an average parasitism rate of 34% (n=61, 2021-2023). Willow Flycatcher breeding success on the Owens River over the past years has apparently been good enough to support a healthy flycatcher population despite Brown-headed Cowbird parasitism. Nonetheless, reducing cowbird parasitism would be a good management strategy given the decline of Willow Flycatcher populations in California. Increasing the number of young birds produced in this population could also increase the dispersal of flycatchers to other areas along the Owens River as well as beyond the Owens River watershed.

Mary J. Whitfield is the Research Director at the Southern Sierra Research Station. She holds a B.S. degree in Wildlife and Fisheries Biology from U.C. Davis, and a M.S. degree in Biology from California State University, Chico. She has also taken post graduate classes at U.C. Santa Barbara. She has over 30 years of fieldwork experience in the U.S. and has also worked in numerous Latin American countries. Her research interests include conservation biology, breeding ecology (particularly with the Willow Flycatcher, Brown-headed Cowbird, and Yellow-billed Cuckoo), behavioral ecology, full lifecycle research, and riparian systems. She has been actively involved with the Partners In Flight Western Working Group (PIF WWG) for over 10 years and is currently leading the PIF WWG Western Motus Initiative.