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 1980 ); mating behavior in California ( Dickinson and Leonard 1996 , Dickinson 1997 , Dickinson and Akre 1998 , Dickinson et al. 2000 , Dickinson 2001 ); parental behavior in Arizona and California ( With and Balda 1990 , Leonard et al. 1994 , Leonard et al. 1995 , Dickinson and Weathers 1999 , Dickinson 2003b ); helping behavior in California ( Dickinson et al. 1996 , Dickinson 2004a , Dickinson 2004b ); foraging habits in Washington and Arizona ( Pinkowski 1979d , Herlugson 1983b ); habitat selection and use in Arizona ( Szaro 1976 , Cunningham et al. 1980a , Brawn 1985 ); environmental effects on breeding and feeding ecology in Arizona and New Mexico ( Szaro 1976 , Brawn 1991 , Fair and Myers 2002b , Germaine and Germaine 2002 , Colestock 2006 , Wightman and Germaine 2006 ); and growth and energetics in California ( 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.