Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2022 annual report

Abstract

Executive Summary Surveys for the endangered Southwestern Willow Flycatcher ( Empidonax traillii extimus ) were done at Marine Corps Base Camp Pendleton (MCBCP or “Base”), California, between May 9 and July 20, 2022. All of MCBCP’s historically occupied riparian habitat (core survey area) was surveyed for flycatchers in 2022. None of the non-core survey area was surveyed in 2022. Eight transient Willow Flycatchers of unknown subspecies were observed on three of the five drainages surveyed in 2022. Willow Flycatchers were not detected at Fallbrook or Pilgrim Creeks. Transients occurred in a range of habitat types, including mixed willow (Salix spp.) riparian, riparian scrub, and upland scrub habitat. Exotic vegetation, primarily poison hemlock ( Conium maculatum ), was present in most of the flycatcher locations. In 2022, for the second time since monitoring began in 2000, resident Southwestern Willow Flycatchers were not detected on Base. The decline was not isolated to MCBCP; similar declines have been documented across California in recent years. The one uniquely banded adult female flycatcher present during the 2021 breeding season did not return to MCBCP in 2022. None of the transients observed during surveys were seen to carry bands. From 2000 to 2022, adult annual survival of Southwestern Willow Flycatchers on MCBCP was 60±3 percent, whereas first-year survival was 20±3 percent. A conspecific attraction study was initiated on Base in 2018 and repeated annually through 2022; flycatchers were not observed near automated playback units in 2022.