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—2019 data summary

We surveyed for Least Bell’s Vireos (Vireo bellii pusillus; vireo) along the San Luis Rey River, between College Boulevard in Oceanside and Interstate 15 in Fallbrook, California (middle San Luis Rey River), in 2019, and we surveyed and conducted nest monitoring for Southwestern Willow Flycatchers (Empidonax traillii extimus; flycatcher) in a survey area where breeding had historically been documented on the middle San Luis Rey River, in 2019. Surveys were conducted from April 11 to June 24 (vireo) and from May 16 to July 15 (flycatcher). We found 179 vireo territories, at least 124 of which were occupied by pairs. Vireo territories increased by 100 percent within the portion of the middle San Luis Rey River that burned as a result of a wildfire in 2017. In contrast, vireo territories increased by 5 percent within the unburned portion of the middle San Luis Rey River.

Vireos used five different habitat types in the survey area: mixed willow riparian, willow-cottonwood, riparian scrub, willow-sycamore, and upland scrub. Fifty-two percent of the vireos were detected in habitat characterized as mixed willow, and 92 percent of the vireos were detected in habitat with greater than 50 percent native plant cover. Of the 12 banded vireos detected in the survey area, 5 were resighted with a full color-band combination. One adult female with a unique color-band combination immigrated to the middle San Luis Rey River from Marine Corps Base Camp Pendleton (MCBCP). Five other vireos with single (natal) federal bands were recaptured, identified, and color banded in 2019. Two vireos with a single dark blue federal band, indicating that they were banded as nestlings on the lower San Luis Rey River (LSLR), could not be recaptured for identification. The five natal vireos that were recaptured on the middle San Luis Rey River dispersed from 1.4 to 8.3 kilometers (km) from their natal territories. Banded vireos with a known age ranged from 1 to 11 years old.

One resident flycatcher was observed in the survey area in 2019. The resident flycatcher (male) was detected in a territory of mixed willow habitat with greater than 50 percent native plant cover. He was detected as a single male from May 16 to July 17, 2019, and no evidence of pairing or nesting was observed. The male flycatcher with a unique color-band combination occupied the same territory in 2018 and 2019.