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Literature Review

Least Bell's Vireo

Vireo bellii

USFWS: Endangered
CDFG: Endangered

Background

Distribution, Abundance and Trends. The least Bell's vireo inhabits riparian woodland habitats along the riverine systems of southern California, primarily in San Diego, Santa Barbara, and Riverside counties. They also breed in northern Baja California and are seen in migration in southern Baja California. This vireo species occurs at sites with two primary features: 1) a dense shrub cover within 1 to meters (3 to 6 feet) of the ground, where nests are typically placed, and 2) a dense, stratified canopy for foraging (Goldwasser 1981, USFWS 1998). Typical riparian habitats are those which may include cottonwoods (Populus fremontii), oak woodlands, and a dense understory of species such as willow (Salix spp.), mulefat (Baccharis salicifolia), and California wild rose (Rosa californica); in desert areas, arrow weed (Pluchea sericea) and wild grape (Vitis girdiana) may be dominant species in these riparian woodlands.

The least Bell's vireo was formerly known to inhabit dense willow thickets along streams throughout California's Sacramento and San Joaquin Valleys, from Red Bluff south, from coastal areas inland to the foothills of the Sierra Nevada, and in Owens and Death Valleys. Currently, U.S. populations are known only from Santa Barbara County and southern California. Major causes of the decline are cowbird parasitism and destruction of riparian habitats. In San Diego County, however, significant population increases in the period from 1986 to 1996 are primarily due to management of local cowbird populations (USFWS 1998).

The least Bell's vireo is known to occur as a breeding bird in Chino Canyon and in Andreas Canyon. Other suitable breeding habitat may occur in Millard Canyon, Whitewater Canyon, Mission Creek, Palm Canyon, and Murray Canyon, at Oasis de Los Osos, at the Willow Hole-Edom Hill Preserve/ACEC, along the Whitewater River near the Salton Sea and at Dos Palmas. Breeding and other habitat in Andreas, Palm and Murray Canyons is on the Agua Caliente Indian Reservation, is part of the Indian Canyons Heritage Park, and is not included in this Plan. Some Bell's vireos, particularly if sighted near the Salton Sea or at other locations on the valley floor, could be subspecies arizonae, but the Plan will address all Bell's vireo habitat as if occupied by subspecies pusillus.

Least Bell's vireos also migrate through the Plan area en route to other breeding areas. In migration, they may use desert fan palm oasis woodland, mesquite hummocks, mesquite bosque, arrowweed scrub, desert dry wash woodland, southern sycamore-alder riparian woodland, Sonoran cottonwood-willow riparian forest, and southern arroyo willow riparian forest.

The least Bell's vireos typically arrive in southern California to breed from mid-March to early April and remain until late September. Most birds spend the winter in southern Baja California and Mexico. During the breeding season, male vireos establish and defend territories; they maintain a stubborn attachment to these sites throughout the breeding season. Nests are constructed in dense thickets of willow or mulefat, one to two meters from the ground. These vireos may also make their nests in other riparian tree and shrub species.

Threats and Limiting Factors. The most significant threats to the least Bell's vireo in the Plan area are nest parasitism by brown-headed cowbird, destruction of habitat as a result of flood control activities, invasion of non-native plants in riparian habitats, and degradation of habitat as a result of edge effects related to human activities. Brown-headed cowbird parasitism has been described as a primary cause for the decline of least Bell's vireos in central and northern California as well as southern California. The decline in breeding populations of lowland riparian passerine species, including the least Bell's vireo, along with other small, insectivorous, open-cup nesting birds -- among them the yellow warbler and southwestern willow flycatcher -- is well documented. It has been suggested that because the least Bell's vireo is most restricted to lowland riparian forests where cowbird parasitism is likely to be greatest, this species has suffered the largest aggregate reduction in numbers. Parasitized vireo pairs either desert the nest or raise the young cowbird at the expense of their own young. Human activities, including golf courses and agriculture, attract cowbirds thereby increasing the threat to least Bell's vireos.

Special Considerations. Reduction of cowbird populations in least Bell's vireo habitat has been shown to substantially benefit this species. The predominance of golf courses and agricultural areas, which both provide habitat for the cowbird may make control of this non-native bird difficult.