Montana Field Guide

Small thrush, 16.5 to 19 cm in length, 24 to 31 g mass. Plumage sexually dimorphic. Adult male has bright, deep, cobalt-blue head (including chin and throat) and upperparts (including wings and tail), with chestnut breast (chestnut coloration may be bisected, laterally, by blue), and frequently chestnut back patches. Remaining underparts grayish, becoming blue-gray on belly and undertail-coverts. Adult female considerably duller and not extensively bluish. Head and throat gray, back gray-brown, abdomen and undertail-coverts grayish, blue in wings and tail. Adult plumages are similar throughout the year (Guinan et al. 2000).

For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.

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Western Bluebirds can usually be found in open coniferous and deciduous woodlands, 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 (Guinan et al. 2000).

Insects during the warmer months, but forages primarily on berries and fruits through the winter. Wintering individuals are especially abundant in years and in areas when mistletoe and juniper berry crops are plentiful (Guinan et al. 2000).

Nests usually found in rotted or previously excavated cavities in trees and snags, or between trunk and bark. Eggs are subelliptical to short subelliptical. Pale blue in color, occasionally white, unmarked. Clutch size typically 5 eggs (range 2 to 6). Near Fortine, egg dates are from May 8 to June 26. The young leave the nest as early as June 9. The latest date for young in the nest is July 7.