The Mountain bluebird, also known as the Arctic bluebird - is a small thrush that is easily identified by the vivid bright blue plumage of the male. In fact, this species has the bluest plumage out of all the North American bluebirds. The female is mainly gray but has a bluish tint, particularly on her wings and tail. She also has a white eye-ring, lacking in the male, and some females have a light reddish-brown color on their breasts and throats. Juveniles look like females but with slightly duller and darker upperparts, and a scaly effect, especially on their throat and breast.
This species breeds as far north as eastern and central Alaska, and south through central and western Canada (western Manitoba and southern Yukon) to areas in the west of the US (east of the Rockies). Winters are spent as far to the south as central Mexico. These birds breed in grassland where there are scattered shrubs and thickets, savannahs, tree-line tundra and clear-cuts and prairie-forest. They need cavities to nest in. They winter in flat grassland that features some trees and bushes, meadows with trees and hedges, lowland, farmland and different types of forest.
During the breeding season in spring and summer, these birds usually are seen alone, or in pairs or - at the end of the breeding season - in family groups of a few members. This species is diurnal and forages from perches, as other bluebirds do, snatching food from the ground, from vegetation, and in mid-air. They are excellent foragers in the air, hovering kestrel-like before they drop onto their prey, or catching insects on the wing. Darting flight and hovering require 8 to 4 times as much energy as does hunting from a perch, therefore they tend to use these methods when food is scarce. The Mountain bluebird is migratory in the north of its range, but most of the population moves in winter to a lower elevation within its breeding range. Movements are also closely linked to the availability of fruit. The birds travel in flocks of as many 30, sometimes more. During migration, they will often stop to feed. During winter, they will form loose flocks with other species of passerine.
Mountain bluebirds form pair bonds which are monogamous. This means that males mate with only one female and females mate with only one male. At the start of the breeding season, males are the first to arrive at the breeding grounds. They select a nest site, being a cavity in a tree or a rock crevice. The females then arrive, and to attract a female, a male flies around the site of the nest while calling. April to September is the breeding season. This species generally produces two broods. Building of the nest is done by the female, sometimes with help from the male, which will guard her or bring some materials. 4-8 eggs of pale blue are laid, and are incubated over 13-14 days. Both parents feed, strongly protect and defend the chicks. During this time they are often very aggressive. The young fledge at 18-21 days, and depend on their parents for another 3 weeks to 2 months.
Mountain bluebirds are quite common and they are not currently threatened. Their populations, however, are declining in places where trees are not big enough to provide natural cavities for nesting, and where agricultural and forestry practices have reduced the number of suitable nest sites. Competition is high for sites among cavity-nesting birds that can’t excavate them by themselves. Mountain, western, and eastern bluebirds compete for nesting boxes in places where their ranges overlap. European starlings, House sparrows, and House wrens are also fierce competitors with bluebirds for nest sites.
According to the What Bird resource, the total number of the Mountain bluebird population is 5,200,000 individuals. The All About Birds resource states that the total breeding population of this species numbers 4.6 million, with 80% in the U.S. for part of the year, 20% breeding in Canada, while 31% winters in Mexico. Overall, currently Mountain bluebirds are classified as Least Concern (LC) on the IUCN Red List and their numbers today are increasing.
These insect-eating birds are helpful in controlling insect populations.