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

Abstract

Executive Summary 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. In 2021, vireo territories decreased by 14 percent in the portion of the middle San Luis Rey River that burned in 2017 and decreased by 2 percent outside of the burned area. Vireos used six different habitat types in the survey area: (1) willow-cottonwood, (2) mixed willow riparian, (3) riparian scrub, (4) upland scrub, (5) willow-sycamore, and (6) non-native. Forty percent of the vireos were detected in habitat characterized as willow-cottonwood, and 97 percent of the vireos were detected in habitat with greater than 50-percent native plant cover. Of the 14 banded vireos detected in the survey area, 3 were resighted with a full color-band combination. There were 10 other vireos with single (natal) federal bands that were recaptured, identified, and color-banded in 2021. One vireo with a single dark blue federal band, indicating that it was banded as a nestling at the lower San Luis Rey River, could not be recaptured for identification. The 10 natal vireos that were recaptured on the middle San Luis Rey River dispersed from 2.0 to 11.7 kilometers from their natal territories. Banded vireos with a known age ranged from 1 to 7 years old. One resident flycatcher, of undetermined breeding status, was observed in the survey area in 2021. The resident flycatcher (male) was detected in a territory of mixed willow habitat with greater than 50-percent native plant cover. He was detected from May 20 to June 4, 2021, and no evidence of pairing or nesting was observed. The male flycatcher was resighted with a unique color-band combination and had occupied the same territory since 2018.