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 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 (U.S. Fish and Wildlife Service, 1986; California Natural Diversity Database, 2026). 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, southwestern Colorado, and possibly 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; Remsen, 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 1991 and by the USFWS in 1995 (U.S. Fish and Wildlife Service, 1995; California Natural Diversity Database, 2026). 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, flycatcher numbers have remained low, unlike vireo numbers, which have increased tenfold since the mid-1980s in response to management practices alleviating threats (U.S. Fish and Wildlife Service, 2006). As of 2023, most of the flycatchers in California are 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, 2002).

Male vireos arrive on breeding grounds in southern California in mid-March. Males 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 often are 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 (Kus and others, 2022).

Male flycatchers begin arriving in southern California in early to mid-May, 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 (U.S. Fish and Wildlife Service, 2002).

The Mojave River Dam is within USFWS designated critical habitat for the flycatcher, and although this area is not designated critical habitat for the vireo, breeding birds have been documented at Mojave River Dam since 2020 (Howell and Kus, 2022, 2025; Singer, 2026). Managed by the U.S. Army Corps of Engineers for flood control, the Mojave River 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 is required to perform surveys and assess activities that might have adverse effects on these federally endangered bird species (U.S. Army Corps of Engineers, 2023). 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 Mojave River Dam in 2025. These data will inform natural resource managers about the status of these endangered species at the Mojave River Dam and help to guide land use and management practices as appropriate to support the species’ survival.

A total of two territorial male vireos were detected at the Mojave River Dam in 2025, both of which were confirmed as paired (tables 1, 2; fig. 2). One vireo pair was south of the dam, and one was north of the dam (fig. 2). One transient vireo (MO02) was detected on May 16, 2025, but was not present on subsequent surveys. Two nests were observed incidentally at one territory (MO01), but the nests were inactive on subsequent visits. No juvenile vireos were detected in 2025 (table 1). No banded vireos were observed in 2025 (table 2). No territorial flycatchers were observed. One transient flycatcher of unknown subspecies was detected in May but was not seen on subsequent surveys, indicating that the bird did not establish a territory (table 1).

Vireos and flycatchers used two habitat types within the study area: riparian scrub and willow-cottonwood (table 3). Several willow species were dominant in vireo territories, including arroyo or red willow and narrowleaf willow (table 3). Exotic vegetation was sparse in vireo and flycatcher locations (table 3).

In 2025, we documented two vireo territories at the Mojave River Dam study area. The population of vireos at the Mojave River Dam has fluctuated between one and four territories since 2020 (Howell and Kus, 2022, 2025; Singer, 2026).

Although no territorial flycatchers were detected in 2025, the detection of one transient flycatcher may indicate that the habitat at the Mojave River Dam provides suitable habitat for migrating flycatchers, especially within the more hydric Deep Creek section of the study area.