Western Bluebirds are small thrushes that breed throughout much of the western United States, Mexico, and southwestern Canada. Males have brilliant blue plumage on their heads, wings, and tails, rust colored breasts, and, frequently, chestnut back patches; females are duller and have more brown and gray in their feathers. Insectivorous during the warmer months, individuals forage primarily on berries and fruits through the winter; wintering individuals are especially abundant in years and in areas when mistletoe (Phoradendron spp.) and juniper (Juniperus spp.) berry crops are plentiful.

Western Bluebirds are socially monogamous. Both parents usually care for young, but they also seek matings outside the pair bond, with the result that offspring are not always related to the attendant male. Helpers at the nest occur in some populations and are frequently adult male relatives, often putative sons of the resident pair, or juveniles from earlier broods.

Western Bluebirds can usually be found in open, parklike forests, edge habitats, burned areas and where moderate amounts of logging have occurred, provided a sufficient number of larger trees and snags remain to provide nest sites and perches. Unlike Eastern (Sialia sialis) and Mountain (Sialia currucoides) bluebirds, Western Bluebirds do not favor large, open meadows. Clear-cutting, snag removal, fire suppression, and any changes in land use that cause open forest and edge habitat to be diminished adversely affect Western Bluebird populations.

Apparent declines in numbers of this species in the Pacific Northwest and British Columbia, especially in regions west of the Cascade Range, have generated concern. In response, bluebird enthusiasts in Washington, Oregon, and British Columbia have established trails of nest boxes in an effort to reestablish local breeding populations.

Systematic studies of Western Bluebird ecology and behavior are limited. Information on some topics is anecdotal or missing altogether. Key studies include: breeding biology and phenology in Washington ( Herlugson, C. J. (1980). Biology of sympatric populations of Western and Mountain bluebirds. Phd Thesis, Washington State Univ., Pullman. Close Herlugson 1980); mating behavior in California ( Dickinson, J. L. and M. L. Leonard. (1996). Mate attendance and copulatory behavior in Western Bluebirds: evidence of mate guarding. Anim. Behav. no. 52:981-992. Close Dickinson and Leonard 1996, Dickinson, J. L. (1997). Male detention affects extra-pair copulation frequency and pair behavior in Western Bluebirds. Anim. Behav. no. 53:561-571. Close Dickinson 1997, Dickinson, J. L. and J. J. Akre. (1998). Extrapair paternity, inclusive fitness, and within-group benefits of helping in Western Bluebirds. Molecular Ecol. no. 7:95-105. Close Dickinson and Akre 1998, Dickinson, J. L., K. Kraaijeveld and F. Smit-Kraaijeveld. (2000). Specialized extrapair mating display in Western Bluebirds. Auk no. 117 (4):1078-1080. Close Dickinson et al. 2000, Dickinson, J. L. (2001). Extrapair copulations in western bluebirds (Sialia mexicana): female receptivity favors older males. Behavioral Ecology and Sociobiology no. 50 (5):423-429. Close Dickinson 2001); parental behavior in Arizona and California ( With, K. A. and R. P. Balda. (1990). Intersexual variation and factors affecting parental care in Western Bluebirds: a comparison of nestling and fledgling periods. Can. J. Zool. no. 68:733-742. Close With and Balda 1990, Leonard, M. L., K. L. Teather, A. G. Horn, W. D. Koenig and J. L. Dickinson. (1994). Provisioning in Western Bluebirds is not related to offspring sex. Behav. Ecol. no. 5:455-459. Close Leonard et al. 1994, Leonard, M. L., J. L. Dickinson, A. G. Horn and W. Koenig. (1995). An experimental test of offspring recognition in Western Bluebirds. Auk no. 112:1062-1064. Close Leonard et al. 1995, Dickinson, J. L. and W. W. Weathers. (1999). Replacement males in the western bluebird: opportunity for paternity, chick-feeding rules, and fitness consequences of male parental care. Behavioral Ecology and Sociobiology no. 45 (3-4):201-209. Close Dickinson and Weathers 1999, Dickinson, J. L. (2003b). Male share of provisioning is not influenced by actual or apparent loss of paternity in western bluebirds. Behavioral Ecology no. 14 (3):360-366. Close Dickinson 2003b); helping behavior in California ( Dickinson, J. L., W. D. Koenig and F. A. Pitelka. (1996). Fitness consequences of helping behavior in the Western Bluebird. Behav. Ecol. no. 7:168-177. Close Dickinson et al. 1996, Dickinson, J. L. (2004a). A test of the importance of direct and indirect fitness benefits for helping decisions in western bluebirds. Behavioral Ecology no. 15 (2):233-238. Close Dickinson 2004a, Dickinson, J. L. (2004b). Facultative sex ratio adjustment by western bluebird mothers with stay-at-home helpers-at-the-nest. Animal Behaviour no. 68:373-380. Close Dickinson 2004b); foraging habits in Washington and Arizona ( Pinkowski, B. C. (1979d). "Foraging ecology and habitat utilization in the genus Sialia." In The role of insectivorous birds in forest ecosystems., edited by J. G. Dickson, R. N. Conner, R. R. Fleet, J. C. Kroll and J. A. Jackson, 165-190. New York: Academic Press. Close Pinkowski 1979d, Herlugson, C. J. (1983b). Hover-foraging by Western Bluebirds. Murrelet no. 64:58-60. Close Herlugson 1983b); habitat selection and use in Arizona ( Szaro, R. (1976). Population densities, habitat selection and foliage use by the birds of selected ponderosa pine forest areas in the Beaver Creek watershed. Phd Thesis, Northern Arizona Univ., Flagstaff. Close Szaro 1976, Cunningham, J. B., R. P. Balda and W. S. Gaud. (1980a). Selection and use of snags by secondary cavity-nesting birds of the ponderosa pine forest. U.S. For. Serv. Close Cunningham et al. 1980a, Brawn, J. D. (1985). Population biology, community structure, and habitat selection of birds in ponderosa pine habitat. Phd Thesis, Northern Arizona Univ., Flagstaff. Close Brawn 1985); environmental effects on breeding and feeding ecology in Arizona and New Mexico ( Szaro, R. (1976). Population densities, habitat selection and foliage use by the birds of selected ponderosa pine forest areas in the Beaver Creek watershed. Phd Thesis, Northern Arizona Univ., Flagstaff. Close Szaro 1976, Brawn, J. D. (1991). Environmental effects on variation and covariation in reproductive traits of Western Bluebirds. Oecologia no. 86:193-201. Close Brawn 1991, Fair, J. M. and O. B. Myers. (2002b). The ecological and physiological costs of lead shot and immunological challenge to developing western bluebirds. Ecotoxicology no. 11 (3):199-208. Close Fair and Myers 2002b, Germaine, H. L. and S. S. Germaine. (2002). Forest restoration treatment effects on the nesting success of Western Bluebirds (Sialia mexicana). Restoration Ecology no. 10 (2):362-367. Close Germaine and Germaine 2002, Colestock, K. L. (2006). Landscape scale assessment of contaminant effects on insectivorous birds. Master's Thesis, Utah State Univ., Logan. Close Colestock 2006, Wightman, C. S. and S. S. Germaine. (2006). Forest stand characteristics altered by restoration affect Western Bluebird habitat quality. Restoration Ecology no. 14 (4):653-661. Close Wightman and Germaine 2006); and growth and energetics in California ( Mock, P. J. (1990b). Energetics of growth and reproduction of the Western Bluebird, Sialia mexicana. Phd Thesis, Univ. of California, Los Angeles. Close Mock 1990b). More information is needed about Western Bluebird vocalizations, migratory physiology and habits, winter ecology, survivorship and mortality, and dispersal habits. Comparative studies of Western Bluebirds to Eastern and Mountain bluebirds in flexible responses to climate and habitat change may also prove interesting.