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Photo by Brian E. Small, Utah Div. Wildlife Res..
Gray Vireo is a summer inhabitant of the hot, dry, pinyon-juniper woodlands of the southwestern United States. It is threatened by the continued destruction of its breeding habitat for livestock grazing. Due in part to its preference for inhospitable habitats, relatively little is known about population trends, susceptibility to cowbird parasitism, and other aspects of Gray Vireo's biology.
Gray Vireo is a drab, medium-sized vireo with a small bill. It is gray overall, with the upperparts being a darker gray and the underparts being more of a grayish white. The only noticeable field marks on this bird are a thin white eye ring, a white lore between the eye and the bill, and a weak white wing-bar.
The heart of Gray Vireo's breeding range extends across a broad swath of the "Four Corners" states of Arizona, New Mexico, Utah, and Colorado. It can also be found breeding in limited areas of southern California, northern Baja California, and western Texas. A number of Audubon Important Bird Areas (IBAs) provide important nesting habitat for Gray Vireo, including Colorado's Canyons of the Ancients National Monument IBA, which hosts a minimum of 500 breeding pairs, and California's San Diego Peaks and East Mojave Peaks IBAs. This species winters from southern Arizona into northern Mexico, in southern Baja California, and in western Texas.
Breeding Bird Survey data for the western United States from 1980 to 1996 showed an annual increase of 6.4% for Gray Vireo. It has been suggested, however, that at least part of this trend might be explained by an improved ability of field workers to detect this easily overlooked species.
Gray Vireo breeds in some of the hottest, driest areas of the American Southwest, favoring dry thorn scrub, chaparral, and pinyon-juniper and oak-juniper scrub, in arid mountains and high plains scrubland. This species forages in thickets, taking most of its prey from leaves, twigs, and branches of small trees and bushes. Its diet on the breeding grounds consists of a variety of arthropods, including large grasshoppers, cicadas, and caterpillars. Winter diet differs based on locality--birds found in western Texas are primarily insectivorous, while those wintering in southern Arizona and adjacent northern Mexico feed mainly on fruit.
The nest of Gray Vireo is cuplike and hangs freely from the fork of horizontal branches. It consists of woven grasses, mesquite, or juniper bark, and is lined with fine grass, hair, and thistle-down. Both sexes build the nest, and both birds incubate a typical clutch of four eggs for about two weeks. Chicks are fed by both parents, and leave the nest about two weeks after hatching. A breeding pair can raise two broods per year. Gray Vireo nests are parasitized by Brown-headed Cowbirds, although the incidence of brood parasitism seems to be high only in California. Gray Vireos often respond to cowbird eggs by abandoning the nest (even if it contains a full clutch of vireo eggs) and then constructing a new nest.
The Gray Vireo is a short-distance migrant, leaving the northern parts of its breeding range by early autumn. It is still not known, though, where birds breeding in eastern Colorado and western Texas winter, nor is it known where birds wintering in western Texas spend the breeding season.
The primary threat to Gray Vireo is loss of habitat. Habitat loss in southern California since the 1940s is blamed for a contraction of the species' range in that area, as well as a decrease in numbers. In the heart of the bird's range, in Arizona, Utah, New Mexico, and Colorado, there has been substantial clearing of pinyon-juniper woodlands to create foraging area for domestic livestock; more than one million acres of such woodland were cleared in Arizona between 1950 and 1961. Pinyon-juniper woodland is also degraded by the widespread practice of cutting trees for firewood.
The clearing of woodland for use by livestock can also lead to an increase in Brown-headed Cowbirds, which parasitize Gray Vireo nests. While the impact of cowbird parasitism on Gray Vireo reproductive success across the vireo's range is not fully understood, a potential increase in cowbird numbers is at least cause for concern.
In September 1995, the U.S. Fish and Wildlife Service listed Gray Vireo as a migratory, nongame songbird whose current status was of management concern in the Southwest and Great Plains/Rocky Mountains regions. In 1998, Partners in Flight (http://www.partnersinflight.org/) drafted its Bird Conservation Plan, in which it provided conservation objectives, management recommendations, and research priorities for Gray Vireo across its range. In Arizona, which is home to both breeding and wintering birds, objectives and recommendations included: maintaining at least five distinct viable populations; preserving pinyon-juniper woodland with shrubby understory; protecting pinyon-juniper habitat by avoiding fire suppression, which results in overly dense tree stands; discouraging the spread of livestock grazing; and educating the government and the public about the value of pinyon-juniper woodlands.
What Can You Do?
Audubon's Important Bird Area program is a vital tool for the conservation of Gray Vireo as well as other species. To learn more about the Important Bird Areas programs in Colorado, California, and other states where the species occurs and how you can help, visit: http://www.audubon.org/bird/iba/.
Volunteers are crucial to the success of programs that monitor the status of populations of Gray Vireo and other bird species. Audubon's Christmas Bird Count (CBC) is one of the longest-running citizen-science monitoring programs in the world and has helped to follow changes in the numbers and distribution of Gray Vireo. To learn more about the CBC and how you can participate, visit: http://www.audubon.org/bird/cbc.
Barlow, J.C., S. N. Leckie, and C. T. Baril. 1999. Gray Vireo (Vireo vicinior). In The Birds of North America, No. 447 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Kaufman, Kenn. Lives of North American Birds. Boston: Houghton Mifflin,
Sibley, David Allen. The Sibley Guide to Birds. New York: Alfred A. Knopf, 2000.