Distribution and abundance of Southwestern Willow Flycatchers ( Empidonax traillii extimus ) on the Upper San Luis Rey River, San Diego County, California—2023 data summary

Abstract

Executive Summary We surveyed for Southwestern Willow Flycatchers ( Empidonax traillii extimus ; flycatcher) along the upper San Luis Rey River near Lake Henshaw in Santa Ysabel, California, in 2023. Surveys were completed at four locations: three downstream from Lake Henshaw, where surveys previously occurred from 2015 to 2022 (Rey River Ranch [RRR], Cleveland National Forest [CNF], Vista Irrigation District [VID]), and one at VID Lake Henshaw (VLH) that has been surveyed annually since 2018. There were a minimum of 74 territorial flycatchers detected at 1 location (VLH), and 12 transient flycatchers of unknown subspecies detected at 2 locations (CNF and VLH). At VLH, we detected a minimum of 31 males, 40 females, and 3 flycatchers of unknown sex. In total, 51 territories were established, containing 40 pairs and 11 flycatchers of undetermined breeding status (8 males and 3 flycatchers of unknown sex). Of the 40 pairs, 9–11 pairs were monogamous (1 male and 1 female), and 29–31 pairs were polygynous (1 male paired with more than 1 female). For the first time since annual surveys began in 2015, no territorial flycatchers were detected downstream from Lake Henshaw. Brown-headed cowbirds ( Molothrus ater ; cowbird) were detected at all four survey locations. No banded flycatchers were detected during surveys. Flycatchers used three habitat types in the survey area: (1) mixed willow riparian, (2) willow-cottonwood, and (3) oak-sycamore. Of the flycatcher locations, 86 percent were in habitat characterized as mixed willow riparian, and 95 percent were in habitat with greater than 95-percent native plant cover. Exotic vegetation was not prevalent in the survey area. There were five nests incidentally located during surveys: one failed, one was seen with eggs on the last visit, and the outcome of the remaining three nests was unknown. One of these nests was parasitized by cowbirds, and a second nest was suspected to contain a cowbird nestling. Adult flycatchers in two territories were observed feeding cowbird fledglings. No juvenile flycatchers were detected during surveys.