Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2020 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 4 and July 31, 2020. All of MCBCP’s historically occupied riparian habitat (core survey area) was surveyed for flycatchers in 2020. Additionally, one-fifth of the unoccupied riparian habitat (non-core survey area) was surveyed for flycatchers. Thirteen transient Willow Flycatchers of unknown subspecies were observed on four of the seven drainages surveyed in 2020. No Willow Flycatchers were detected at Fallbrook Creek, Pilgrim Creek, or San Mateo Creek. Transients occurred in a range of habitat types, including mixed willow ( Salix spp.) riparian, riparian scrub, willow-sycamore ( Platanus racemosa ) or willow-cottonwood ( Populus fremontii ) dominated riparian vegetation, and upland scrub. Exotic vegetation, primarily poison hemlock ( Conium maculatum ), was present in most flycatcher locations. The resident population of Southwestern Willow Flycatchers on MCBCP declined 33 percent from three individuals in 2019 to two individuals in 2020. In 2020, the resident Southwestern Willow Flycatcher population on Base consisted of one male and one female. No single males or non-territorial floaters were observed in 2020. Overall, one territory was established consisting of one monogamous pair. Resident flycatchers were restricted to the Santa Margarita River, and distribution was limited to the Pueblitos breeding area. All resident flycatchers were located in mixed willow riparian habitat. Nesting was initiated in late May and continued into early August. Three nesting attempts were documented, of which 33 percent (1/3) were successful. Predation and substrate failure accounted for the two nest failures. Two fledglings were produced, yielding a seasonal productivity of two young/pair. No instances of Brown-headed Cowbird ( Molothrus ater ) parasitism were observed. Flycatchers placed nests in two plant species: native sandbar willow ( Salix exigua ) and exotic poison hemlock. One hundred percent of resident birds that were present at MCBCP in 2020 were banded in previous years; no unbanded birds were detected. Of the three uniquely banded adult flycatchers present during the 2019 breeding season, 100 percent (1/1) of males and 50 percent (1/2) of females returned to MCBCP in 2020, and both banded flycatchers returned to the same breeding area they occupied in 2019. None of the seven nestlings banded in 2019 returned to MCBCP in 2020, and none were detected off Base. Six nestlings from two nests were banded in 2020; only two survived to fledging. From 2000 to 2020, overall adult survival of Southwestern Willow Flycatchers on MCBCP was 60 percent, while first-year survival was 20 percent. A conspecific attraction study initiated on Base in 2018 and repeated annually through 2020 found that 100 percent of breeding flycatchers detected in 2020 settled close to automated playback units.