Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2021 annual report

Executive Summary

The purpose of this report is to provide the Marine Corps with an annual summary of abundance, breeding activity, demography, and habitat use of the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) at Marine Corps Base Camp Pendleton (MCBCP). Surveys for the flycatcher were completed at MCBCP between May 5 and July 31, 2021. All of MCBCP’s historically occupied riparian habitat (core survey area) was surveyed for flycatchers in 2021. Additionally, one-fifth of the non-historically occupied riparian habitat (non-core survey area C) was surveyed for flycatchers. Twenty-four transient Willow Flycatchers of unknown subspecies were observed on five of the seven drainages surveyed in 2021. No Willow Flycatchers were detected at French or Las Flores Creeks. Transients occurred in a range of habitat types, including mixed willow (Salix spp.) riparian, riparian scrub, willow-sycamore (Platanus sp.) dominated or oak (Quercus spp.) sycamore-dominated riparian, and non-native-dominated riparian habitat. Exotic vegetation, primarily poison hemlock (Conium maculatum), was present in most flycatcher locations.

The resident population of Southwestern Willow Flycatchers on MCBCP declined 50 percent, from two individuals in 2020 to one individual in 2021. In 2021, the resident Southwestern Willow Flycatcher population on MCBCP consisted of one unpaired female occupying one territory. No males were observed in 2021. The resident flycatcher population was restricted to the Santa Margarita River, and distribution was limited to the Pueblitos breeding area. The resident flycatcher territory was located in mixed willow riparian habitat.

Nesting was initiated in late May and continued into late July. Two nesting attempts were documented, neither of which were successful. Infertile eggs likely accounted for both nest failures. No instances of Brown-headed Cowbird (Molothrus ater) parasitism were observed. The female flycatcher placed her nests in native sandbar willow (Salix exigua) and used the same nest location for both nesting attempts.

Of resident birds that were present at MCBCP in 2021, 100 percent were banded in previous years; no unbanded birds were detected. Of the two uniquely banded adult flycatchers (one male, one female) present during the 2020 breeding season, 50 percent (one female) returned to MCBCP in 2021. The banded female returned to the same breeding area and territory she occupied in 2020. Neither of the two nestlings banded in 2020 returned to MCBCP in 2021, and neither were detected off Base. From 2000 to 2021, adult over-winter survival of Southwestern Willow Flycatchers on MCBCP was 60±3 percent (mean±standard error [SE]), and first-year survival was 20±3 percent.

A conspecific attraction study that used automatic playback units to broadcast flycatcher vocalizations in order to encourage flycatchers to settle on MCBCP was initiated in 2018 and repeated annually through 2021. The single resident flycatcher (female) detected in 2021 settled close to an automated playback unit.