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, 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 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).

Male vireos arrive on breeding grounds in southern California in mid-March. Male vireos are territorial and vocally conspicuous, frequently singing their diagnostic primary song from exposed perches throughout the breeding season. Females arrive about 1–2 weeks after males and are more secretive, but they 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 about 1 meter (m) above the ground. Nesting occurs from early April through July, but adult 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).

Vireo breeding habitat consists of dense, shrubby vegetation characteristic of early stages of ecological succession, usually near river channels or other water (Kus and others, 2022). Nests are placed in a wide variety of small trees, shrubs, and less frequently, tall annual herbaceous vegetation (Houston and others, 2024).

The Southwestern Willow Flycatcher (Empidonax traillii extimus; flycatcher) is one of four subspecies of Willow Flycatcher in the United States, with a breeding range that includes 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 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 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 half-dozen pairs.

Male flycatchers begin arriving in southern California in early to mid-May, and 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 leave their breeding territory in mid-August and early September to migrate to their wintering grounds in Central America and northern South America (U.S. Fish and Wildlife Service, 2002).

Flycatcher breeding habitat is characterized as patches of dense riparian vegetation along rivers and streams, interspersed with small openings, open water, or areas of sparse vegetation. Vegetation species composition varies across the range, but most breeding habitat includes tree or shrub cover that is at least 3 m tall, with patches of dense vegetation within 3–4 m of the ground. In addition, flycatchers typically nest near areas of standing water or saturated soil (U.S. Fish and Wildlife Service, 2002; Sogge and others, 2010).

The Coastal California Gnatcatcher (Polioptila californica californica; gnatcatcher) is a small gray songbird restricted to low stands of coastal and inland sage scrub on arid hillsides, mesas, and washes. Gnatcatchers are year-round residents throughout southern California from Ventura County southward into northern Baja California, Mexico (Vandergast and others, 2022). Limited naturally by the patchy distribution of its habitat, gnatcatcher populations have become further fragmented in recent decades by urbanization, habitat degradation, and stochastic events, such as wildfire (U.S. Fish and Wildlife Service, 2025b). As a result, gnatcatcher populations have diminished in size and distribution and occur largely as islands in a matrix of generally unsuitable habitat. The gnatcatcher was listed as federally threatened in 1993 (U.S. Fish and Wildlife Service, 2025b).

Gnatcatcher breeding season begins mid-February and lasts through the end of August, with peak nesting activity from mid-March through mid-May (U.S. Fish and Wildlife Service, 2019). Territorial males often sing or call from exposed perches (Atwood and Bontrager, 2020). Nest sites are selected by males and are typically 1 m above the ground in California sagebrush (Artemisia californica) or California buckwheat (Eriogonum fasciculatum). Both sexes participate in nest-building, although males initiate and perform most of the construction (Atwood and Bontrager, 2020). Like vireos and flycatchers, gnatcatchers are subject to brood parasitism by cowbirds (Braden and others, 1997).

Carbon Canyon Dam falls within the breeding range of all three species. Additionally, it is a USFWS-designated critical habitat of the gnatcatcher (U.S. Fish and Wildlife Service, 2025a). The Carbon Canyon Dam was built for flood control and completed in 1961. Managed by the U.S. Army Corps of Engineers, the 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 listed bird species. The purpose of this report is to summarize the results of vireo, flycatcher, and gnatcatcher surveys completed by the U.S. Geological Survey (USGS) at the Carbon Canyon Dam in 2025. These data will inform natural resource managers about the status of these listed species at the Carbon Canyon Dam and guide land use and management practices as appropriate to support the species’ survival.

Methods

Study Area

The study area is in the Puente and Chino Hills along Carbon Canyon Creek and Telegraph Canyon Creek upstream from the Carbon Canyon Dam (fig. 1). The Carbon Canyon Dam is in the city of Brea, south of Carbon Canyon Road and east of State Route 57 in Orange County, California. Aside from an artificial lake and some seasonally wet catch basins, the reservoir behind the dam usually is dry and includes Carbon Canyon Creek and Telegraph Canyon Creek. Below the dam, Carbon Canyon Creek flows through a concrete flood control channel. The reservoir encompasses the 50-hectare (ha; 124-acre) Carbon Canyon Regional Park, which includes 24.3 ha (60 acres) of developed land, including grassy areas for picnicking, sports facilities, and a 1.6-ha (4-acre) lake. The undeveloped areas of the park are accessed by trails and include riparian and disturbed coastal sage scrub habitats and a 1.2-ha (3-acre) coast redwood (Sequoia sempervirens) grove. The eastern extent of the study area includes approximately 9.5 ha (23.5 acres) of Chino Hills State Park.

The area surveyed included 2.1 kilometers (km; 1.3 miles [mi]) of Carbon Canyon Creek, 0.5 km (0.3 mi) of Telegraph Canyon Creek, and 26.3 ha (65 acres) of coastal sage scrub in the uplands. The narrow riparian corridors that weave through the site are dominated by willows and mule fat (Baccharis salicifolia). In 2025, Carbon Canyon Creek and Telegraph Canyon Creek had some water in the easternmost part of the survey area but were mostly dry. Water was present in the artificial lake, but the catch ponds above the dam were dry in April when surveys began. The coastal sage scrub is mostly disturbed and consists of non-native annuals such as non-native grasses, poison hemlock (Conium maculatum), and black mustard (Brassica nigra), with a scattering of native sage scrub shrubs such as California sagebrush and California buckwheat. Limited higher-quality sage scrub habitat exists on the hilltops.

Results

A total of 14 territorial male vireos were detected at the Carbon Canyon Dam in 2025, 12 of which were confirmed as paired (tables 1, 2; fig. 2). In addition, one transient vireo (CY14) was detected on May 15, 2025, but was not present on subsequent surveys. Five vireo nests were incidentally observed during surveys; three of these nests were parasitized by cowbirds, and cowbirds were observed throughout the study area. Juvenile vireos were confirmed in two territories (table 1). No banded vireos were observed in 2025 (table 2). No flycatchers or gnatcatchers were detected on surveys (table 1).

Vireos used four habitat types within the study area: (1) mixed willow, (2) riparian scrub, (3) non-native, and (4) upland. Mixed willow was the most commonly used habitat type (table 3). Dominant plant species found at vireo locations included Goodding’s black willow and mule fat. Most vireo locations (12) had greater than 50-percent native vegetation. The most common exotic plant species at vireo locations was poison hemlock (table 3).

Summary

In 2025, we documented 14 vireo territories and one transient bird at the Carbon Canyon Dam study area. The population of vireos within the study area at the Carbon Canyon Dam declined slightly since biologists from the U.S. Geological Survey last performed protocol surveys in 2019 (16 territories; R. Fisher, U.S. Geological Survey, unpub. data, 2019). Cowbirds were observed throughout the survey area, and parasitism was documented at Carbon Canyon Dam in 2025, despite the observation of cowbird trapping at the eastern extent of the study area in Chino Hills State Park.

Although no territorial flycatchers were detected in 2025, transient flycatchers have been detected in previous years, which may indicate that the habitat at the Carbon Canyon Dam provides value as stopover habitat during migration (R. Fisher, U.S. Geological Survey, unpub. data, 2019).

Gnatcatcher territories were absent from Carbon Canyon Dam in 2025, a reduction from two territories in 2018 and four territories in 2019 (R. Fisher, U.S. Geological Survey, unpub. data, 2018, 2019). It is possible that gnatcatchers shifted to coastal sage scrub on surrounding private lands, which was out of detection range for our surveys in 2025.