The Least Bell’s Vireo (Vireo bellii pusillus; vireo) is a small, migratory songbird that breeds in southern California and northwestern Baja California, Mexico, from April through July (Kus and others, 2022). Historically abundant within lowland riparian ecosystems, vireo populations began declining in the late 1900s as a result of multiple anthropogenic factors, including habitat loss and alteration associated with urbanization and agricultural conversion of land adjacent to rivers, the expansion in range of the brood-parasitic Brown-headed Cowbird (Molothrus ater; cowbird), and the introduction of invasive exotic plant species, such as giant reed (Arundo donax), into riparian systems (U.S. Fish and Wildlife Service, 1986, 1998; Franzreb, 1989; Kus, 1998, 1999; Riparian Habitat Joint Venture, 2004; Kus and others, 2022). By 1986, the vireo population in California numbered just 300 territorial males (U.S. Fish and Wildlife Service, 1986).

In response to the considerable decline in vireo numbers in California, the California Fish and Game Commission listed the species as endangered in 1980 (California Natural Diversity Database, 2026), and the U.S. Fish and Wildlife Service (USFWS) followed suit in 1986 (U.S. Fish and Wildlife Service, 1986). Since listing, the vireo population in southern California has rebounded, largely in response to cowbird control and habitat restoration and preservation (Kus and Whitfield, 2005). As of 2006, the statewide vireo population was estimated to be approximately 2,500–3,000 territories (U.S. Fish and Wildlife Service, 2006).

The Southwestern Willow Flycatcher (Empidonax traillii extimus; flycatcher) is one of four subspecies of Willow Flycatcher in the United States, with a breeding range including southern California, Arizona, New Mexico, extreme southern parts of Nevada and Utah, and western Texas (Hubbard, 1987; Unitt, 1987; Browning, 1993). Restricted to riparian habitat for breeding, the flycatcher has declined over the past five decades in response to widespread habitat loss throughout its range and, possibly, brood-parasitism by cowbirds (Wheelock, 1912; Willett, 1912, 1933; Grinnell and Miller, 1944; Remson, 1978; Garrett and Dunn, 1981; Unitt, 1984, 1987; Gaines, 1988; Schlorff, 1990; Whitfield and Sogge, 1999). By 1993, the species was believed to number approximately 70 pairs in California (U.S. Fish and Wildlife Service, 1993) in small, disjunct populations. The flycatcher was listed as endangered by the State of California in 1992 (California Natural Diversity Database, 2026) and by the USFWS in 1995 (U.S. Fish and Wildlife Service, 1993). After listing, population estimates for flycatchers in California increased to 256 territories, with the increase largely attributed to expanded survey efforts rather than population growth at known sites (U.S. Fish and Wildlife Service, 2002). In the 2014 5-year status review, estimates of California flycatcher territories decreased to 172, with declines occurring statewide (Durst and others, 2008; U.S. Fish and Wildlife Service, 2014).

Flycatchers in southern California co-occur with vireos. However, unlike the vireo, which has increased tenfold since the mid-1980s in response to management practices alleviating threats (U.S. Fish and Wildlife Service, 2006), the number of flycatchers has remained low. As of 2023, most of the flycatchers in California were concentrated at two known sites: (1) the Owens River valley in Inyo County (approximately 56 territories; M. Whitfield, Southern Sierra Research Station, written commun., 2023) and (2) the upper San Luis Rey River at Lake Henshaw in San Diego County (approximately 51 territories; Howell and Kus, 2024). Outside of these sites, flycatchers occur as small, isolated populations of one to six pairs (U.S. Fish and Wildlife Service, 2014).

The purpose of this report is to summarize the results of vireo and flycatcher surveys completed by the U.S. Geological Survey (USGS) at the Sepulveda Dam basin, Los Angeles County, California, in 2025. These data will inform natural resource managers about the status of these endangered species at the Sepulveda Dam basin and help to guide land use and management practices as appropriate to support the species’ survival.

A total of 11 territorial male vireos and 3 transient vireos were detected in 2025 (tables 1, 2; fig. 2). Of the 11 territorial vireos, 4 were confirmed as paired, and 7 were of undetermined status. No banded birds were detected, and no juvenile vireos were detected during surveys (table 2). Cowbirds were abundant throughout the project area and detected on every survey. One incident of cowbird parasitism was detected on July 7, 2025, when a vireo pair was observed feeding a cowbird fledgling. No cowbird trapping was done in the Sepulveda Dam project area during the 2025 surveys.

Vireos were found in four habitat types, with 43 percent of vireo territories occurring in mixed willow riparian habitat and 36 percent occurring in willow-cottonwood. Fourteen percent of vireos were detected in riparian scrub, and 7 percent were in willow-sycamore habitat (table 3). Of the vireo territories, 93 percent occurred in habitat comprised of greater than 50-percent native plant cover. The one territory in habitat with less than 50-percent native vegetation was in an area co-dominated by willows and exotic castor bean (Ricinus communis; table 4).

No flycatchers were detected during surveys in 2025.

In 2025, we documented 11 resident vireo territories at the Sepulveda Dam project area. The population of vireos at the Sepulveda Dam project area was consistent with 2022 when 10 vireo territories were documented (Pottinger and Kus, 2023) but still below the 2018 population when 14 vireo territories were documented (Pottinger and Kus, 2019). Cowbirds were observed throughout the project area, and an incident of parasitism was documented during surveys. No cowbird trapping was done at the Sepulveda Dam project area in 2025. Although no flycatchers were detected in 2025, the vegetation at the Sepulveda Dam project area may provide stopover habitat for migrating flycatchers.