Warbling Vireo is a complex of two groups of subspecies in eastern and western North America. These groups may represent at least two distinct species. The two groups differ in size, genetics, vocalizations and ecology (Am. Ornithol. Union 1998, Gardall and Ballard 200).
DISTRIBUTION. During the 1987-1992 field work of the TBBA project, researchers found confirmed breeding evidence for Warbling Vireo in the northeast Panhandle and south of the Red River valley (eastern group). Breeding in the Trans-Pecos mountains represents the western group (Lockwood and Freeman 2004). The confirmed record in latilong block 31102 probably also represents this group. The possible and probable records elsewhere deserve further study to see if they are actual breeding or late migrants and to which group they belong.
Elsewhere in North America Warbling Vireo breeds from southeast Alaska across Canada to Newfoundland, then south through the Appalachian Mountains and the Mississippi River drainage. West of the Great Plains, the range extends south through the higher elevations of the western United States and Mexico to the Isthmus of Tehuantepec. In winter Warbling Vireos in the United State and northern Mexico move south to the south of the breeding range and Central America as far south as Nicaragua. Other breeders witter in southern Baja California (Gardall and Ballard 2000).
SEASONAL OCCURRENCE.Warbling Vireo migrates through the state from March 13 to May 28 and is uncommon to common from mid-April to mid-May. The breeding season is probably from early May to late July. Extreme dates for southbound migrants are July 17 and November 26 and this vireo is uncommon to common from mid-August to late October. Warbling Vireo is apparently rare in winter (Oberholser 1974, Lockwood and Freeman 2004).
BREEDING HABITAT. In Texas Warbling Vireo breeds from near sea level to 2400 m (8000 ft). The eastern race prefers tall deciduous trees in open woodlands, riparian areas, orchards, and parks (Oberholser 1974). In Arizona and Colorado with breeding habitats most similar to those in the Trans-Pecos mountains, most breeding was found in mixed deciduous-coniferous, montane riparian and pine-oak habitats (totaling about 70% of Arizona nests; Averill-Murray 2005) and aspen, riparian deciduous and upland conifer habitats (totaling about 80% of Colorado nests; Barrett 1998). In Mexico Howell and Webb (1995) found Warbling Vireo breeding in humid to semiarid mixed coniferous-deciduous woodlands.
Warbling Vireo often places its nest in a horizontal fork of a slender branch well away from the trunk. The eastern race places its nest high in a tree, while the nest of the western race is usually within 4 m (12 ft) of the ground. The nest, built by both sexes in about a week, is made of bark strips, leaves, greases, feathers and plant down, held together and attached to thee branch with spider silk. The lining is plant stems and animal hair. The rim of the nest overhangs the deep cup. The outside diameter is 9×5 cm (3.5×2 in), height 6 cm (2.4 in), inside diameter and cup depth 5 cm (2 in; Harrison 1979).
The female usually lays 4 (range 3-5) smooth, slightly glossy white eggs (see Harrison  for photo of markings). Both sexes share incubation for about 13-14 days (east), about 12.5 (range 10.5-16) days (west). The male may sing from the nest, aiding in its location. Nestlings leave the nest 13-14 (range 10.5-19) days after hatching. The eastern race is rarely subjected to brood parasitization, the rate is often 50-80% in the west with Brown-headed Cowbird (Molothrus ater) the usual parasite; a Bronzed Cowbird (M. aeneus) was observed being fed by a Warbling Vireo (Harrison 1979, Gardall and Ballard 2000).
STATUS. Lockwood and Freeman (2004) consider Warbling Vireo an uncommon to rare summer resident in the Davis and Guadalupe mountains and a sporadic nester in the Chisos Mountains.The species is common to rare in the eastern Panhandle.
The North American Breeding Bird Survey detected this vireo on only 7 of its 40 km (25mi) routes in Texas. The data are insufficient to provide a meaningful population trend for the state. Statistically significant trends of +1.3% (east) and +1.1% (west) population change per year were obtained for the period 1966-2004 (Sauer et al. 2005). The trends are encouraging for the future of both races of Warbling Vireo in Texas.
Text by Robert C. Tweit (2006)
American Ornithologist’s Union 1998. Checklist of North American birds, 7th ed. Am. Ornithol. Union, Washington, DC.
Averill-Murray, A. 2005. Warbling Vireo (Vireo gilvus). In Arizona breeding bird atlas. pp. 346-347 (T. E. Corman and C. Wise-Gervais, eds.). University of New Mexico Press, Albuquerque.
Barrett, N. M. 1998.Warbling Vireo (Vireo gilvus) . In Colorado breeding bird atlas, pp. 308-309 (H. E. Kingery, ed.). Colorado Bird Atlas Partnership, Denver.
Gardall, T. and G. Ballard. 2000. Warbling Vireo (Vireo gilvus). In The birds of North America, No. 551 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Harrison, H. H. 1979. A field guide to western birds’ nests. Houghton Mifflin, Boston, MA.
Howell, S. N. G. and S. Webb. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, New York.
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.
Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin.
Sauer, J. R., J. E. Hines, and J. Fallon. 2005. The North American Breeding Bird Survey, results and analysis 1966-2004. Version 2005.1. USGS Patuxent Wildlife Research Center, Laurel MD (Web site, http://www.mbr-pwrc.usgs.gov/bbs).