The Gray Vireo is a scrub-foraging inhabitant of some of the hottest, most arid regions of the southwestern United States and adjacent parts of northwestern Mexico (Figure 1). Well camouflaged by its drab gray plumage, this vireo's harsh, three- to four-note song is often the only indication of its presence. Once found, a silent individual may be difficult to distinguish from sympatric gray-colored gnatcatchers (Polioptila spp.), Bushtit (Psaltriparus minimus), Oak Titmouse (Baeolophus inornatus), and Juniper Titmouse (B. ridgwayi) in both summer and winter ranges. Estimation of population numbers and trends for this easily overlooked bird are difficult, and further complicated by its preference for inhospitable and often remote climes.
The first known specimen of the Gray Vireo was taken on 24 May 1865, by Elliott Coues at Fort Whipple (now Prescott), Arizona Territory (Coues 1866). Coues (Coues 1878d) named the bird "Gray Greenlet." Its Latin name, from vicinis, means neighboring or related, and refers to the close resemblance of this species to other species of small gray birds. As he encountered no other individuals of this species at the time, Coues believed the bird to be rare. Nine years passed before new reports were made by Henshaw (Henshaw 1875c), who found Gray Vireos along the Colorado Chiquito River, New Mexico, and at Camp Bowie and Camp Lowell, Arizona. The extent of the breeding and wintering ranges of the Gray Vireo, particularly in the eastern parts of its distribution, has only been slowly established, with the discovery of a breeding population in the Chisos Mountains, Brewster Co., TX, in 1935 (Van Tyne and Sutton 1937), and in adjacent Coahuila, Mexico, as recently as May, 1968 (Barlow and Johnson 1969). The wintering grounds of Gray Vireos breeding in eastern Colorado and western Texas, and the breeding grounds of birds wintering in southwestern Texas and San Luis Potosí, Mexico, remain unknown.
A short-distance migrant, the Gray Vireo leaves the northern parts of its breeding range by early autumn. Two apparently disjunct wintering populations have been studied. Birds wintering in western Texas feed predominantly on insects and actively defend winter territories (JCB). In southwestern Arizona and adjacent Sonora, Mexico, however, wintering birds shift from a largely insectivorous summer diet to a predominately frugivorous diet, especially taking the fruit of the elephant tree (Bursera microphylla). These western Gray Vireos also defend winter territories (Bates Bates 1992b, Bates 1992a).
Studies of breeding behavior in Texas, Arizona, and Colorado (Barlow 1967b, Barlow et al. 1970b, Barlow 1980b, Hutchings and Leukering n.d.), foraging behavior and winter territoriality (Griffin 1986, Barlow and Flood 1990; Bates Bates 1992b, Bates 1992a) and geographic variation in song (Werden 1991) have yielded important information about these life history activities. Further investigation is needed of population numbers and trends-including the impact of brood parasitism and habitat alteration-of the wintering and breeding grounds of Colorado, western Texas, and Mexican populations, and of distinctive geographic variation in disjunct populations.