Southwestern Willow Flycatcher (Empidonax traillii extimus) Surveys and Nest Monitoring in San Diego County, California (ver. 5.0, December 2025)

Data presented are results of surveys and monitoring for Southwestern Willow Flycatcher (Empidonax traillii extimus) in San Diego County, California. These data were collected to assess Southwestern Willow Flycatcher abundance and distribution in San Diego County. Surveys were conducted in various years at 33 locations along 10 drainages in San Diego County, including portions of Agua Hedionda Creek, Cottonwood Creek, Escondido Creek, Los Penasquitos Creek, Otay River, San Diego River, San Dieguito River, San Luis Rey River, Sweetwater River, and Tijuana River. From 2016-19, Southwestern Willow Flycatcher territories were monitored at three locations along the upper San Luis Rey River (Cleveland National Forest, Rey River Ranch, Vista Irrigation District) to determine breeding productivity, survival, and dispersal. Nesting data were compiled to present seasonal productivity for each Southwestern Willow Flycatcher pair. When possible, adults and nestlings in the monitoring area were banded with unique color combinations to identify individuals on subsequent visits to provide data on survivorship and dispersal patterns.