Physical Description
Unlike other bluebird species, male Mountain Bluebirds have no chestnut red on their bodies. The head, back, wings, and tail are a bright sky blue. Males are light blue from the chin to the belly, and grayish-white on the belly and undertail coverts.
Females have brownish gray upperparts. The wings, rump, and tail are a pale or light blue. Females sometimes have a pale reddish throat and breast, but more commonly, the throat and breast are gray brown.
Juveniles look like adult females, but they are darker and less colorful. Their breasts and sides are streaked with brown.
Distribution and Breeding Habitat
Mountain Bluebirds are found in the western parts of Canada and the United States. They are found at elevations above 5,000 feet, and they nest in open areas such as meadows, hayfields, grain fields, savannas, prairies, clear cuts, and the edges of coniferous and deciduous forests.
Diet
Mountain Bluebirds feed on a variety of insects, including beetles, weevils, ants, wasps, cicadas, flies, grasshoppers, katydids, and crickets. Unlike their eastern or western relatives, Mountain Bluebirds rarely eat seeds or berries. Mountain Bluebirds also differ from other bluebirds in that they often hover while foraging.
Pair Formation and Territoriality
Males return to the breeding grounds before the females and establish territories. This happens in late March in the south but not until early April in the north. Females arrive several days to a few weeks later. Initially, pairs aggressively defend a large area around their nest site, but as the breeding season progresses, the size of the defended area decreases and becomes more localized around the nest. Mountain Bluebirds defend territories against others of their own species, or conspecifics, as well as against other bluebird species in areas where their breeding ranges overlap.
Mountain Bluebirds are basically monogamous. But, as with other bluebirds, both males and females sneak copulations with individuals that are not their mates. For this reason, males guard their mates, from the time a pair forms until the female lays her eggs. Pairs typically stay together throughout the season; however, individuals that have had an unsuccessful first nesting attempt may find a new mate for the rest of the breeding season. Some pairs mate for more than one breeding season. As in many species, the fact that many pairs mate with each other year after year probably reflects a male and female fidelity to a particular breeding site rather than to one another.
Nesting Behavior
Nest Building: Mountain Bluebirds are solitary nesters, but pairs may nest in close proximity to others, depending on nest site availability. The beginning of the breeding season varies with latitude; birds in the southern portion of the range begin in early April, birds in northern latitudes begin in late May. The female chooses a nest site, which can be any natural cavity, abandoned woodpecker hole, cliff crevice, or nest box. Like other bluebirds, Mountain Bluebirds compete with House Sparrows and European Starlings for nest sites.
Only the female builds the nest, and it takes her anywhere from a few days to over a week to complete. Working most diligently in the morning, she constructs the nest of grass, weed stems, pine needles, twigs, rootlets, bark, and, sometimes, wool, hair, or feathers. Males are very attentive to their females during the nest-building period and spend most of their energy guarding their mates. Occasionally, they carry nest material to the cavity, but they are not known to actually weave it into the nest.
Egg Laying: The first eggs are laid between late April and early May. Females lay one egg per day until the clutch is complete. The average clutch size is five to six eggs, but there can be as few as four or as many as eight. The eggs are smooth, glossy, unmarked, and are pale blue, bluish-white, or, rarely, white. Because all eggs laid by a female are the same color, any odd-colored eggs in a clutch indicates that another female has laid an egg in that nest, a behavior known as egg dumping.
Incubation: While some sources report that both sexes incubate, incubation is mainly done by the female. Incubation begins when the next-to-last or the last egg is laid and lasts 13 to 14 days. Males often feed their mates during this period, and they continue to do so after the eggs hatch.
Nestling Care:The female broods the nestlings for about a week after they hatch. The male does most of the feeding during that time. When the female ceases her daytime brooding and begins to brood only at night, both sexes start feeding the young equally. The nestlings fledge after 17 to 22 days. Initially, they are sedentary and depend heavily upon their parents for food and protection; however, as they mature, they begin to follow the parents around and actively solicit feedings. The male continues to care for the fledged young when the female begins to re-nest. After three to four weeks, the young are independent.
If a nesting attempt fails, Mountain Bluebirds will renest. They usually raise two broods in a breeding season. The fledged young may assist their parents in raising the next brood, but this behavior is considered rare.
Mountain Bluebirds reuse old nest sites both within a breeding season and in successive breeding seasons. Pairs that successfully raise a brood in a nest box may become faithful to that particular type of nest box. First-year breeding birds tend to nest in boxes identical to their natal box.
Winter Movement and Dispersal
Families form flocks in late summer and merge with other family flocks as the season progresses. Juveniles that fledged early in the season and adults that bred unsuccessfully during the summer also join these flocks. It has not been determined whether the birds stay together in these flocks throughout migration or on their wintering grounds.
Mountain Bluebirds are the most migratory of the three bluebird species. Birds in the northern portion of the range begin to migrate sooner than those living in the southern portion. Mountain Bluebirds migrate to the southern United States and central Mexico, where they inhabit open lowlands, deserts, plains, and grasslands.