Southwestern Willow Flycatcher - Yearly Research Synthesis
(July 15, 2024 – September 30, 2025)
The Southwestern Willow Flycatcher (Empidonax traillii extimus) is a small, migratory songbird that depends on healthy riparian habitats. Recent surveys and research from 2021–2025 show that while some local populations persist, overall numbers remain low and fragmented across southern California and the Southwest (Howell & Kus, 2022, 2024, 2025; Allen & Kus, 2023; Houston et al., 2023, 2024).
Distribution and Abundance
Lake Henshaw continues to be a vital stronghold for SWFL, though the population declined from 74 territorial birds in 2023 to 47 in 2024 (Howell & Kus, 2024). Downstream areas see few residents, mostly transient birds passing through. Other coastal sites, like Marine Corps Base Camp Pendleton and Carlsbad Preserve, have very limited resident flycatchers, highlighting ongoing challenges in restoring populations outside core habitats (Allen & Kus, 2023; Houston et al., 2023, 2024).
Breeding and Nesting
At Lake Henshaw in 2024, researchers found 22 nests across 13 territories, with nine successfully fledging young (Howell & Kus, 2025). Many nest failures were due to predation or parasitism by Brown-headed Cowbirds (Molothrus ater), a long-term challenge for this species (McCreedy, 2021). Past efforts to manage cowbirds have reduced impacts in some areas, but parasitism continues to limit reproductive success (Howell & Kus, 2022, 2025).
Habitat Use and Vegetation Associations
Southwestern Willow Flycatchers favor dense, native riparian vegetation, especially mixed willow communities with high canopy cover. In 2024, nearly all birds at Lake Henshaw were found in areas with mostly native plants (Howell & Kus, 2025), underscoring the importance of healthy, undisturbed habitats. Invasive tamarisk (Tamarix spp.) removal and restoration programs help promote native vegetation, but outcomes vary depending on water availability and management methods (Goetz, 2023; Gaffke et al., 2022; Mahoney et al., 2024).
Threats and Conservation Outlook
The species faces habitat loss, cowbird parasitism, changes to water systems, and the impacts of climate change (Forester et al., 2023). Warmer temperatures and increased drought could raise nest failure rates, but the flycatcher’s genetic diversity and mobility provide some resilience if protected habitats remain intact. Conservation strategies focus on protecting key breeding areas like Lake Henshaw, restoring riparian corridors, managing cowbirds, and continuing careful monitoring (Howell & Kus, 2025; SANDAG, 2025). These combined efforts are crucial to help the Southwestern Willow Flycatcher survive and thrive in the region.
References
Allen, L. D., & Kus, B. E. (2023). Southwestern Willow Flycatcher (Empidonax traillii extimus) surveys at the City of Carlsbad Preserve, San Diego County, California—2022 data summary (U.S. Geological Survey Data Report 1172). U.S. Geological Survey. https://doi.org/10.3133/dr1172
Forester, B. R., Day, C. C., Ruegg, K., & Landguth, E. L. (2023). Evolutionary potential mitigates extinction risk under climate change in the endangered Southwestern Willow Flycatcher. Journal of Heredity, 114(4), 341–353. https://doi.org/10.1093/jhered/esac067
Gaffke, A. M., Dudley, T. L., Bean, D. W., Drus, G. M., Johnson, M. J., Knutson, A. E., Weaver, D. K., Sing, S. E., Orr, B. K., & Thompson, D. C. (2022). Tamarix biological control in North America. In R. G. Van Driesche, R. L. Winston, T. M. Perring, & V. M. Lopez (Eds.), Contributions of classical biological control to the U.S. food security, forestry, and biodiversity (pp. 329–355). U.S. Department of Agriculture, Forest Service.
Goetz, A. R. B. (2023). Plant community response to control of a dominant invader at multiple scales (Doctoral dissertation, University of Denver). University of Denver Digital Commons. https://digitalcommons.du.edu/etd/2243
Houston, A., Allen, L. D., Mendia, S. M., & Kus, B. E. (2023). Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2022 Annual Report (U.S. Geological Survey Open-File Report 2023-1040). U.S. Geological Survey. https://doi.org/10.3133/ofr20231040
Houston, A., Allen, L. D., Mendia, S. M., & Kus, B. E. (2024). Least Bell’s Vireos and Southwestern Willow Flycatchers at the San Luis Rey Flood Risk Management Project Area in San Diego County, California—Breeding activities and habitat use—2023 Annual Report (U.S. Geological Survey Open-File Report 2024-1040). U.S. Geological Survey.
Howell, S. L., & 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 (U.S. Geological Survey Data Report 1158). U.S. Geological Survey. https://doi.org/10.3133/dr1158
Howell, S. L., & 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). U.S. Geological Survey. https://doi.org/10.3133/dr1194
Howell, S. L., & 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). U.S. Geological Survey. https://doi.org/10.3133/dr1212
Howell, S. L., & Kus, B. E. (2024). Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2022 Annual Report (U.S. Geological Survey Open-File Report 2023-1080). U.S. Geological Survey. https://doi.org/10.3133/ofr20231080
Howell, S. L., & 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). U.S. Geological Survey. https://doi.org/10.3133/ofr20251001
Howell, S. L., Kus, B. E., & Preston, K. L. (2025). Science support for recovery of Southwestern Willow Flycatcher (Empidonax traillii extimus) on conserved lands in San Diego County (U.S. Geological Survey Cooperator Report prepared for the San Diego Association of Governments TransNet Environmental Mitigation Program). U.S. Geological Survey. https://doi.org/10.13140/RG.2.2.34430.91200
Mahoney, S. M., Stein, A. C., Stahlke, A. R., Özsoy, A. Z., Esquivel, F. L., & Johnson, M. J. (2024). New locality for the tamarisk (Tamarix spp.) biological agent Diorhabda spp. in southeastern Arizona. The Southwestern Naturalist, 68(3), 228–232. https://doi.org/10.1894/0038-4909-68.3.228
McCreedy, C. (2021). Amargosa Canyon Songbird Project 2021 Progress Report. Amargosa Conservancy.
Pottinger, R. E., & Kus, B. E. (2023). Least Bell’s Vireo (Vireo bellii pusillus) and Southwestern Willow Flycatcher (Empidonax traillii extimus) surveys in the Sepulveda Dam Basin, Los Angeles County, California—2022 data summary (U.S. Geological Survey Data Report 1177). U.S. Geological Survey. https://doi.org/10.3133/dr1177
Sferra, S. J., Sogge, M. K., Whitfield, M. J., & Kus, B. E. (2003). Singing behavior of Southwestern Willow Flycatchers in Arizona. Studies in Avian Biology, 26(1), Article 21.