The western bluebird (Sialia mexicana) is a small North American thrush and was formally described by the English naturalist William John Swainson in 1832.
The adult male is 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. The adult female has a duller blue body, wings, and tail, a gray throat, a dull orange breast, and a gray belly and undertail coverts. Both sexes have a thin straight bill with a fairly short tail. Immature birds have duller colors than the adults, and have spots on their chest and back.
The western bluebird can be readily distinguished from the two other species in the bluebird genus. The western bluebird has a blue (male) or gray (female) throat, the eastern bluebird (Sialia sialis) has an orange throat, and the mountain bluebird (Sialia currucoides) lacks orange color anywhere on its body.