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. Historically abundant in 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, 2020). By 1986, the vireo population in California numbered just 300 territorial males (U.S. Fish and Wildlife Service, 1986).

In response to the dramatic numeric decline of vireos in California, the California Fish and Game Commission listed the species as endangered in 1980, and the U.S. Fish and Wildlife Service (USFWS) followed suit in 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). Restricted to riparian habitat for breeding, the flycatcher has declined in recent 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 and by the USFWS in 1995.

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. Currently, most flycatchers in California are concentrated at one site, the upper San Luis Rey River near Lake Henshaw in San Diego County (Howell and Kus, 2021). Outside of this site, flycatchers occur as small, isolated populations of one to six pairs. Data on the distribution and demography of the flycatcher, as well as identification of factors limiting the species, are critical information needs during the current stage of recovery planning (Kus and others, 2003; Kus and Whitfield, 2005).

Male vireos arrive on breeding grounds in southern California in mid-March. Male vireos are vocally conspicuous and frequently sing their diagnostic primary song from exposed perches throughout the breeding season. Females arrive approximately 1–2 weeks after males and are more secretive but are often seen early in the season traveling through habitat with the males. The female, with the male's help, builds an open cup nest in dense vegetation approximately 1 meter (m) above the ground. Nesting occurs from early April through July, but adults and juvenile birds remain on the breeding grounds into late September and early October before migrating to their wintering grounds in southern Baja California, Mexico.

Male flycatchers begin arriving in southern California at the end of April, whereas females arrive approximately 1 week later. While on the breeding grounds, males sing repeatedly from exposed perches. Once the pair bond is established, the female builds an open cup nest that is usually placed in a branch fork of a willow (Salix spp.) or plant with a similar branching structure approximately 1–3 m above the ground. Adults usually depart from their breeding territory in mid-August and early September to their wintering grounds in central America and northern South America.

Although it is outside of the published breeding range as of 2015 for the species (U.S. Fish and Wildlife Service, 2021), vireos are known to occur at the San Antonio Dam near Upland, California. The San Antonio Dam also falls just outside of the documented breeding range for the flycatcher, but the species may still be detected at the San Antonio Dam. Managed by the U.S. Army Corps of Engineers for flood control, the San Antonio Dam requires regular operational maintenance, including debris, sediment, and vegetation removal and management. As mandated by the USFWS, the U.S. Army Corps of Engineers must conduct surveys and assess activities that might have adverse effects on these federally endangered bird species. The purpose of this report is to summarize the results of vireo and flycatcher surveys conducted by the U.S. Geological Survey (USGS) at the San Antonio Dam in 2021. These data will inform natural resource managers about the status of these endangered species at the San Antonio Dam and guide modification of land use and management practices as appropriate to ensure the species’ continued existence.

No territorial vireos were detected, although we did detect one singing male on the final survey in July (table 1; fig. 2). Because no vireos were detected during the first three survey periods, this individual was considered to be a transient (table 2). The vireo was not banded.

No territorial flycatchers were detected; however, we detected one individual on May 27, 2021 (table 1; fig. 2). This individual was not observed on subsequent surveys and was considered to be a transient (table 2). We were unable to get a visual of the flycatcher’s legs; therefore, it is unknown if it was banded.

The vireo and flycatcher were found in different habitat types within the San Antonio Dam basin. The vireo was observed in mule fat-dominated riparian scrub that included more than 50-percent native plant cover, whereas the flycatcher was found in habitat almost exclusively dominated by non-native tamarisk (table 3).

In 2021, we documented one vireo using the habitat at the San Antonio Dam late in the breeding season. It is possible that this individual held a territory elsewhere in the region and was using the habitat at the dam during a southbound migration. The habitat at San Antonio Dam was last occupied by breeding vireos in 2018, when one territorial pair was reported (M. Madden, U.S. Geological Survey, unpub. data, 2018). The 2018 male was color-banded and was observed again, in 2020, 45 kilometers away at the Mojave River Dam (M. Madden, U.S. Geological Survey, unpub. data, 2020). Though the San Antonio Dam may not be occupied by breeding vireos every year, it is likely an important component in the regional patchwork of riparian habitat within an urban landscape.

Although no breeding flycatchers were detected in 2021, the vegetation at the San Antonio Dam appears to provide important stopover habitat for migrating flycatchers.