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—2018 data 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 2018. Surveys were conducted from April 17 to July 16 (vireo) and from May 16 to July 27 (flycatcher). We found 148 vireo territories, at least 90 of which were occupied by pairs. Six additional transient vireos were detected. Vireos used six different habitat types in the survey area: mixed willow, willow-cottonwood, riparian scrub, willow-sycamore, upland scrub, and non-native habitat. Forty-one percent of the vireos were detected in habitat characterized as mixed willow, and 97 percent of the vireos were detected in habitat with greater than 50 percent native plant cover. Of 10 banded vireos detected in the survey area, 5 had been given full color-band combinations prior to 2018. Four other vireos with single (natal) federal bands were recaptured, identified, and color banded in 2018. One vireo with a single dark blue federal band, indicating that it was banded as a nestling on the lower San Luis Rey River, could not be recaptured for identification. 

One resident flycatcher and eight transient flycatchers of unknown subspecies were observed in the survey area in 2018. The resident flycatcher (male) was detected in a territory of mixed willow habitat with greater than 95 percent native plant cover. He was detected as a single male from May 24 to July 17, 2018, and no evidence of pairing or nesting was observed. The male flycatcher, detected with a single natal band, was recaptured, identified, and given a unique color combination in 2018. The male flycatcher was originally banded as a nestling on the middle San Luis Rey River in 2016. The eight transient flycatchers were detected from May 25 to June 8, 2018, in mixed willow riparian, willow-cottonwood, and riparian scrub habitat with greater than 95 percent native plant cover.