The Western Bluebird (Sialia mexicana) is a small thrush, approximately 15 cm (5.9 in) to 18 cm (7.1 in) in length.
Adult males are bright blue on top and on the throat with an orange breast and sides, a brownish patch on back, and a gray belly and undertail coverts. Adult females have a duller blue body, wings, and tail than the male, a gray throat, dull orange breast, and a gray belly and undertail coverts. Immature Western Bluebirds have duller colors than the adults, they also have spots on their chest and back.
They are sometimes confused with other bluebirds, however they can be distinguished without difficulty. The Western Bluebird has a blue (male) or gray (female) throat, the Eastern Bluebird has an orange throat, and the Mountain Bluebird lacks orange color anywhere on its body.
Nesting habitat 
Western Bluebird breeding habitat is semi-open country, excluding desert areas. The year round range includes California, southern Rocky Mountains, Arizona, and New Mexico in the U.S., and as far south as the states of Oaxaca and Veracruz in Mexico. The summer breeding range extends as far north as the Pacific Northwest, British Columbia, and Montana. Northern birds can migrate to the southern parts of the range; southern birds are often permanent residents.
They nest in cavities or in nest boxes, competing with Tree Swallows, House Sparrows, and European Starlings for natural nesting locations. Because of the high level of competition, House sparrows often attack Western Bluebirds for their nests. The attacks are made both in groups or alone. Attacks by Starlings can be reduced if the nesting box opening is kept to 1 1/2" to avoid takeover.
Nest type and habitat comparison 
In restored forests Western Bluebirds have a higher probability of successfully fledging young than in untreated forests, but they are at greater risk of parasitic infestations. The effects on post-fledging survival are unknown. Western Bluebirds have been found to enjoy more success with nest boxes than in natural cavities. They started egg laying earlier, had higher nesting success, lower predation rates, and fledged more young in boxes than in cavities but they did not have larger clutches of eggs.
The eggs are commonly 2-8 which average 20.8 x 16.2mm. Eggs are oval shape with a smooth and glossy shell. They are pale blue to bluish white and sometimes white in color. Nestlings remain in a nest about 19–22 days before fledging.
Rearing of young 
In a good year, the parents can rear two broods; with four to six eggs per clutch. According to genetic studies, 45% of Western Bluebirds' nests carried young that were not offspring of the male partner. In addition, Western Bluebirds will help their parents raise a new brood after their own nest fails. Western Bluebirds are also helped by other birds belonging to a different species altogether.
These birds wait on a perch and fly down to catch insects, sometimes catching them in midair. They mainly eat insects and berries. During the breeding season, Bluebirds are very helpful with pest control in the territory surrounding the nest.
Similar species 
- BirdLife International (2012). "Sialia mexicana". IUCN Red List of Threatened Species. Version 2012.1. International Union for Conservation of Nature. Retrieved 16 July 2012.
- Powell, Hugh; Barry Jessie, Haber Scott, Parke-Houben Annetta (2011). "Western Bluebird" (Web Article). The Cornell Lab of Ornithology. Retrieved October 5, 2011.
- Germaine, H., Germaine, S. (2002) Restoration Ecology; Restoration Ecology 10(2), 362–367
- Sibley, D. A. 2003. The Sibley Field Guide to Birds of North America. Chanticleer Press, New York.
- Western Bluebird page at Cornell
- Western Bluebird Species Information from Bluebird Information and Awareness
- Western Bluebird videos, photos & sounds on the Internet Bird Collection
- Western Bluebird Information at USGS Patuxent Bird Identification Infocenter
- Western Bluebird slow motion hovering video
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