Introduction

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, 2020). 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, 2020). By 1986, the vireo population in California had declined to just 300 territorial males (U.S. Fish and Wildlife Service, 1986).

In response to the considerable decline in numbers 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; 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 and by the USFWS in 1995. 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 these threats (U.S. Fish and Wildlife Service, 2006), the number of flycatchers has remained low. As of 2023, most of the Southwestern Willow 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.

The Hansen Dam Basin contains suitable breeding habitat for the vireo and is within USFWS-designated critical habitat for the flycatcher (U.S. Fish and Wildlife Service, 2025). Breeding vireos have been documented in the Hansen Dam Basin over the past two decades (Griffith Wildlife Biology, 2009; Pottinger and Kus, 2019; R. Fisher, U.S. Geological Survey, unpub. data, 2020). Managed by the U.S. Army Corps of Engineers for flood control, the Hansen 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. The purpose of this report is to summarize the results of vireo and flycatcher surveys completed by the U.S. Geological Survey (USGS) along a 4-kilometer stretch of the Big Tujunga Creek upstream from Hansen Dam (Hansen Dam Basin) in Los Angeles County, California (fig. 1). These data will inform natural resource managers about the status of these endangered species in the Hansen Dam Basin and guide land use and management practices as appropriate to support the species’ survival.

Methods

Biologists from the USGS surveyed for vireos and flycatchers in the Hansen Dam Basin and Big Tujunga Creek following standard survey techniques (U.S. Fish and Wildlife Service, 2001; Sogge and others, 2010). Four vireo surveys were completed between April 17 and July 2, 2025, and three flycatcher surveys were completed between May 20 and July 2, 2025. Observers walked slowly through or adjacent to suitable riparian habitat, listening and searching for vireos and flycatchers, systematically playing a recording of a vireo or flycatcher song to elicit a territorial response. Surveys typically began at sunrise and were completed by early afternoon, depending on wind and weather conditions. Vireo and flycatcher surveys were completed by USGS biologists Alexandra Houston, Scarlett Howell, and Suellen Lynn under Federal 10(a)1(A) Recovery Permit ESPER0004050_0.3.

For each vireo or flycatcher encountered, observers recorded age (adult or juvenile), sex, breeding status (paired or undetermined), and whether or not the bird was banded. A male was considered paired if a female was detected visually with the male, by hearing vocalizations unique to mated birds, or by observing breeding behavior (for example, food carry, a nest, or dependent juveniles in the territory). A vireo or flycatcher was considered transient if detected only once. Vireo and flycatcher locations were recorded using Esri Field Maps (Esri, 2025) on mobile phones with built-in Global Positioning System to determine geographic coordinates (World Geodetic System of 1984). Because multiple subspecies of flycatchers may be encountered during surveys, we refer to flycatchers in tables and figures as Willow Flycatchers (Empidonax traillii) to include all subspecies.

Dominant native and exotic plants were recorded, and percent cover of exotic vegetation was estimated using categories of less than 5 percent, 5–50 percent, 51–95 percent, and greater than 95 percent. The overall habitat type within the territory was specified according to the following categories:

Results

A total of 62 territorial male vireos were detected in 2025 (table 1; fig. 2). Of the 62 males, 51 were confirmed as paired, and 32 juveniles were detected. We also detected two transient adult vireos. No banded birds were detected.

Least Bell’s Vireos were found in five habitat types with 77 percent of vireo locations in mixed willow riparian habitat (table 2). Seventeen percent of vireos were detected in riparian scrub, 3 percent were in willow-cottonwood habitat, 2 percent were in willow-sycamore habitat, and 2 percent were in upland scrub. Most vireos (95 percent) used habitat largely composed of native vegetation (greater than 50-percent native vegetation). Willows were the dominant plants at 84 percent of vireo locations, with Goodding’s black willow dominant at 66 percent of locations (table 3).

We detected 18 transient Willow Flycatcher of unknown subspecies in the Hansen Dam Basin (fig. 3; table 4). All flycatchers were detected on May 20, 2025, and not the following day where survey areas overlapped. No flycatchers remained to breed at the site. Most transient Willow Flycatchers (78 percent) used mixed willow riparian habitat (table 5), and all Willow Flycatchers were detected in habitat comprised of greater than 50-percent native plant cover. Willows were the dominant species at most flycatcher locations (94 percent of locations; table 6).

Summary

In 2025, we documented 62 resident vireo territories in the Hansen Dam Basin study area. The population of vireos in the Hansen Dam Basin has declined since surveys in 2018 (77 territories; Pottinger and Kus, 2019) and 2020 (73 territories; R. Fisher, U.S. Geological Survey, unpub. data, 2020). Although the number of vireo territories has remained stable in the central (37–38 territories) and eastern (11–13 territories) sections, vireo numbers have dropped 55 percent in the western section, from 29 in 2018, to 23 in 2020, and 13 in 2025. At least three brush fires have been documented within the Hansen Dam Basin since surveys were completed in 2018, all of which were in the western section (August 2018 and May 2022 [National Interagency Fire Center, 2025] and May or June 2025 [S. Howell, U.S. Geological Survey, oral commun., 2025]). Additionally, the western section was notably drier in 2025 than in 2018. In 2018, parts of the western section were inundated with water and impassable during surveys, whereas these areas were easily navigable and dry in 2025 (S. Howell, U.S. Geological Survey, oral commun., 2025). Although the riparian vegetation in the Hansen Dam Basin has been altered by brush fires, encampments, and recreation (hiking, bicycling, and horse trails), the confirmation of nests and juveniles during our surveys indicates that the Hansen Dam Basin retains suitable vireo breeding habitat.

Although no territorial flycatchers were detected in 2025, several transient flycatchers were detected during one survey at Hansen Dam. In 2025, we also surveyed for flycatchers at three other U.S. Army Corps of Engineers sites (Carbon Canyon Dam, Mojave Dam, and Sepulveda Basin, B. Kus, U.S. Geological Survey, unpub. data, 2025), and we only detected one transient flycatcher (at Mojave Dam). The detection of several transient flycatchers indicates that the habitat in the Hansen Dam Basin provides suitable habitat for migrating flycatchers.