Distribution and Abundance of Least Bell's Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Middle San Luis Rey River, San Diego County, Southern California in 2021

We surveyed for Least Bell's Vireos (Vireo bellii pusillus; vireo) and Southwestern Willow Flycatchers (Empidonax traillii extimus; flycatcher) along the San Luis Rey River, between College Boulevard in Oceanside and Interstate 15 in Fallbrook, California (middle San Luis Rey River), in 2021. Surveys were conducted from April 13 to July 14 (vireo) and from May 18 to July 13 (flycatcher). We found 180 vireo territories, at least 125 of which were occupied by pairs. The vireo population size decreased by 6 percent from 2020 to 2021. One resident flycatcher, of undetermined breeding status, was observed in the survey area in 2021. He was detected from May 20 to June 4, 2021, and no evidence of pairing or nesting was observed. These data support the following publication: Allen, L.D. and Kus, B.E., 2022. Distribution and abundance of Least Bell's Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Middle San Luis Rey River, San Diego County, southern California--2021 Data summary (No. 1147). US Geological Survey.