Introduction

Western Bluebirds are small thrushes that breed throughout much of the western United States, Mexico, and southwestern Canada. Males are easily identified by the brilliant blue plumage of their heads, wings, and tails, their 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, this species forages 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, this species does not favor large, open meadows. Western Bluebirds are adversely impacted by clear-cutting, snag removal, fire suppression, and any changes in land use that cause open forest and edge habitat to be diminished.

In recent years, there has been concern about apparent declines in numbers of this species in the Pacific Northwest and British Columbia, especially in regions west of the Cascade Range. 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 few. 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); parental behavior in Arizona and California (With and Balda 1990, Leonard et al. 1994, Leonard et al. 1995); helping behavior in California (Dickinson et al. 1996); foraging habits in Washington and Arizona (Pinkowski 1979, Herlugson 1983); habitat selection and use in Arizona (Szaro 1976, Cunningham et al. 1980, Brawn 1985); environmental effects on breeding and feeding ecology in Arizona (Szaro 1976, Brawn 1991); and growth and energetics in California (Mock 1990). More information is needed about Western Bluebird vocalizations, migratory physiology and habits, winter ecology, survivorship and mortality, and dispersal habits.