cut areas. Along roadsides, they seek out nest boxes or nesting cavities that face away from roads. Mountain Bluebirds winter at lower elevations—in meadows, hedgerows, prairies, and flat grasslands with few scattered trees and bushes, pinyon-juniper and oak-juniper woodlands, and agricultural areas. They avoid the most arid desert habitats.
Mountain Bluebirds eat mostly insects, especially during breeding season. Beetles, grasshoppers, and especially caterpillars top the menu. In winter they go after small fruits, seeds, and insects when available. Commonly eaten plant items include grapes, currants, elderberries, cedar berries, and the seeds of sumac, mistletoe, and hackberry. Spiders are also an important part of the adult diet. Nestlings are fed primarily beetles and grasshoppers.
- Clutch Size
- 4–8 eggs
- Number of Broods
- 1-2 broods
- Egg Length
- 0.7–1 in
- Egg Width
- 0.6–0.7 in
- Incubation Period
- 13 days
- Nestling Period
- 18–21 days
- Egg Description
- Pale blue to bluish white, rarely pure white, and paler than those laid by other bluebirds.
- Condition at Hatching
- Unfeathered, helpless.
Males sometimes enact a kind of symbolic nest-building—miming the act of bringing nesting material to the cavity, but actually carrying nothing, or else dropping their burdens en route. The female builds the insulated nest by herself, usually working hardest in the early morning. She entirely fills the cavity floor with coarse, dry grass stems and other vegetation, hollowing out a cup just large enough to allow her to cover her eggs snugly, with a maximum interior diameter of about 2 inches. The cup is usually greater than 2 inches deep, and placed as far as possible from the entrance hole. Cavity size determines the nest’s exact external dimensions. The female lines the cup with finer plant material, such as fine grass stems and narrow strips of soft bark, and also in some cases with wool or feathers. The whole process can take several days to more than a week. Mountain Bluebirds often reuse nest cavities within and between breeding seasons, and accumulating nesting material can pile up to the level of the entrance hole.
Males scout out possible nest cavities; females choose. Because studies have focused on nest boxes, ornithologists know relatively little about Mountain Bluebirds’ preferences among natural nesting cavities. Males show interest in all cavities within their territory, from knotholes to small rock fissures. Generally they prefer dry cavities in open grassland within 3 feet of the ground, with entrances oriented away from approaching storms. They will also nest on cliffs and buildings.
Mountain Bluebirds forage from perches, like other bluebirds, to snatch food from ground, vegetation, and mid-air. But they are also excellent aerial foragers, hovering kestrel-like before dropping onto prey, or hawking insects on the wing. Hovering and darting flight require four to eight times as much energy as hunting from a perch, so the birds tend to use these techniques when food is scarce. Courting males sing near a nest cavity, flying back and forth between the cavity and a perch near the female. Once a female chooses a cavity, she may become so focused on nesting activities that human observers can approach closely. As breeding season winds down, flocks of 30 or more Mountain Bluebirds begin to form. Each postbreeding flock centers on one or more families with dependent fledglings, later joined by unattached adult birds who failed to reproduce that year. As the last fledglings become mobile, these postbreeding flocks may wander out of sight for periods of days or weeks, returning to visit their nesting areas for a few hours or days, until eventually they disappear from the territory.
Mountain Bluebirds are fairly common and their populations have been stable or slightly declining over the last half-century, according to the North American Breeding Bird Survey. Partners in Flight estimates the global breeding population at about 4.6 million, with 80 percent spending some part of the year in the U.S., 20 percent in 