The Western bluebird (Sialia mexicana) is a small North American thrush. It was first described by the English naturalist William John Swainson in 1832 and has six recognized subspecies.
The adult male of this species 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.
Western bluebirds live in the west of North America (Canada and USA), south to northern Baja California and the central part of Mexico. Northern populations fly south for the winter, while those in the south of the range often are resident, except for seasonal altitudinal movements. This species lives in open conifer forests, forest edges, farmland, and streamside groves where there are scattered trees with grassy areas to forage.
Western bluebirds migrate for short distances. From July until October, they migrate to winter nesting sites, while from February to March they return to the spring nesting sites. Birds at high elevations travel to lower elevations during winter to search for food. Western bluebirds hunt by diving down from perches to catch prey. Sometimes, they catch prey in the air or probe insects from leaves. They beat large prey against a branch or the ground before eating them. During winter, they live in small family groups. These kin groups give protection from predators, with the group's size depending on the availability of food. To provide protection, males use the group to find a mate or birds as helpers at their nest. After winter, the birds migrate to higher elevations for mating or to help at another nest. During the breeding season, these birds are very territorial. Western bluebirds use a range of calls to communicate. They make a mellow “few”, which extends in brief song to “few few fawee”. The “few” or “kew” call, and the “che-check” calls are for location between mates during the breeding season. The defense call when there is an intruder, is a kind of “squawk” uttered by the male.
Western bluebirds are monogamous, with males and females forming long-term bonds, though they are not always faithful. They are cooperative breeders, with helper birds helping the parents raise young. Courtship displays involve the male singing and fluttering in front of the female, with his wings half open and his tail fanned. He may preen the female and offer food to her. His territory is for the purpose of mating, nesting, and feeding. The breeding season is from May to July. Nests are situated in natural cavities, such as woodpecker holes or dead trees or, they may be in nest boxes. The female builds the nest, with the male helping sometimes. 4 to 6 eggs of pale blue are laid, 1 or 2 clutches per season. Incubation is by the female and is for about 12 to 18 days, while she is fed by the male. Chicks are altricial when they hatch, and the female broods them, fed again by the male. Sometimes her mate stays near the nest while she is searching for food, however, he doesn’t brood the chicks. But both parents feed their young, which fledge at around 2 to 3 weeks. They reach reproductive maturity when they are one year old.
These birds are not endangered but their habitat is threatened from extensive logging and the growth of forests due to the prevention of natural fires, as well as development and grazing that have reduced habitat availability. In appropriately wooded habitat, dead trees may be removed in people's effort to clean up; however, this limits places where cavity nesters such as bluebirds can find nest sites. Also, aggressive, non-native European starlings and House sparrows, which are also cavity nesters, may move into many of the sites that might otherwise be used by Western bluebirds.
According to the IUCN Red List, the total population size of the Western bluebird is 7.1 million mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are increasing.
Being insectivorous, these birds affect insect populations in their range. They aslo spread the seeds from berries they eat and thus help recover their native ecosystem.