Gray Vireo Vireo vicinior Scientific name definitions

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, E. (1866). List of the birds of Fort Whipple, Arizona. Proceedings of the Academy of Natural Sciences of Philadelphia 18:39-100. Close ). Coues (Coues 1878d Coues, E. (1878). The Birds of the Colorado Valley: A Repository of Scientific and Popular Information Concerning North American Ornithology, Part I. Goverment Printing Office, Washington, DC, USA. Close ) 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 Henshaw, H. W. (1875). Report upon the ornithological collections made in portions of Nevada, Utah, California, Colorado, New Mexico, and Arizona during the years 1871, 1872, 1873, and 1874, by H. W. Henshaw. In Report Upon Geographical and Geological Explorations and Surveys West of the One Hundredth Meridian (G. M. Wheeler, Editor), U. S. Government Printing Office, Washington, D.C., U.S.A. pp. 131-508. Close ), 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 Van Tyne, J., and G. M. Sutton (1937). The birds of Brewster County, Texas. Miscellaneous Publications of the University of Michigan Museum of Zoology 37:1–115. Close ), and in adjacent Coahuila, Mexico, as recently as May, 1968 (Barlow and Johnson 1969 Barlow, J. C. and R. R. Johnson. (1969). The Gray Vireo: Vireo vicinior Coues (Passeriformes: Vireonidae) in the Sierra del Carmen, Coahuila, Mexico. Canadian Journal of Zoology 47:151-152. Close ). 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, J. M. (1992b). Winter territorial behaviour of Gray Vireos. Wilson Bulletin 104:425-433. Close , Bates 1992a Bates, J. M. (1992a). Frugivory on Bursera microphylla (Burseraceae) by wintering Gray Vireos (Vireo vicinior, Vireonidae) in the coastal deserts of Sonora, Mexico. Southwestern Naturalist 37:252-258. Close ).

Studies of breeding behavior in Texas, Arizona, and Colorado (Barlow 1967b Barlow, J. C. (1967b). Nesting of the Black-capped Vireo in the Chisos Mountains, Texas. Condor 69:605-608. Close , Barlow et al. 1970b Barlow, J. C., R. D. James, and N. Williams (1970). Habitat Co-occupancy Among Some Vireos of the Subgenus Vireo (Aves: Vireonidae). Canadian Journal of Zoology 48:395–398. Close , Barlow 1980b Barlow, J. C. (1980). Patterns of Ecological Interactions Among Migrant and Resident Vireos on the Wintering Grounds. In Migrant Birds in the Neotropics: Ecology, Behavior, Distribution, and Conservation (A. Keast and E. S. Morton, Editors). Smithsonian Institution Press, Washington, DC, USA. pp. 79–107. Close , Hutchings and Leukering n.d. Hutchings, S. and T. Leukering. Nesting ecology and behavior of the Gray Vireo in Western Colorado. Unpubl. manuscript. Close ), foraging behavior and winter territoriality (Griffin 1986 Griffin, B. G. (1986). Habitat correlates of foraging behavior at two levels of temporal resolution in the Gray Vireo (Vireo vicinior). Master's Thesis, Univ. of Toronto, Toronto, Canada. Close , Barlow and Flood 1990 Barlow, J. C. and N. Flood. (1990). "Factors influencing bird population dynamics in the northern Chihuahuan Desert region." In Third symposium on resources of the Chihuahuan Desert-U.S. and Mexico., edited by M. Powell, 81-88. Alpine, TX: Chihuahuan Desert Res. Inst. Close ; Bates Bates 1992b Bates, J. M. (1992b). Winter territorial behaviour of Gray Vireos. Wilson Bulletin 104:425-433. Close , Bates 1992a Bates, J. M. (1992a). Frugivory on Bursera microphylla (Burseraceae) by wintering Gray Vireos (Vireo vicinior, Vireonidae) in the coastal deserts of Sonora, Mexico. Southwestern Naturalist 37:252-258. Close ) and geographic variation in song (Werden 1991 Werden, A. H. (1991). The implications of geographic and interseasonal variation in the song of the Gray Vireo (Vireo vicinior). Master's Thesis, Univ. of Toronto, Toronto, Ontario. Close ) 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.