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The Western bluebird (Sialia mexicana) is a small thrush, about 15 to 18 cm (5.9 to 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, a 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, but 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.
Western bluebird breeding habitat is semiopen country, excluding desert areas. The yearround range includes California, the southern Rocky Mountains, Arizona, and New Mexico in the United States, 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 1.5 in (38 mm) diameter 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. They have been found to enjoy more success with nest boxes than in natural cavities. They started egg-laying earlier, had higher nesting success and 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 two to eight per clutch, with average size 20.8 mm × 16.2 mm (0.82 in × 0.64 in). Eggs are oval in 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 to 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, they 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.
- BirdLife International (2012). "Sialia mexicana". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- 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.