- 6.3–7.9 in
- 1.1 oz
- Slightly larger than a Cassin’s Finch, considerably smaller than an American Robin.
- Merlebleu azuré (French)
- Azulejo pálido (Spanish)
- Historically, the Mountain Bluebird depended for nest sites on forest tree cavities excavated by woodpeckers. Today, many Mountain Bluebirds breed in artificial nest boxes, which tend to be situated in more open areas and have smaller openings to keep out marauders and bad weather. Most of what we know about Mountain Bluebirds comes from studies of these human-made nesting sites.
- A female Mountain Bluebird pays more attention to good nest sites than to attractive males. She chooses her mate solely on the basis of the location and quality of the nesting cavity he offers her—disregarding his attributes as a singer, a flier, or a looker.
- A male Mountain Bluebird frequently feeds his mate while she is incubating and brooding. As the male approaches with food, the female may beg fledgling-style—with open beak, quivering wings, and begging calls. More often, she waits until her mate perches nearby, then silently flicks the wing farthest from him—a signal that usually sends him off to find her a snack.
- Mountain Bluebirds compete fiercely with other cavity-nesters over nest sites. Early spring arrival at nesting grounds, for example, helps them take possession of choice cavities before Tree Swallows can appropriate them. Northern Flickers sometimes enlarge the entrance holes of nest boxes before discovering the box is too small for their own use—rendering the boxes permeable to weather and competitors such as European Starlings.
- The longest-lived Mountain Bluebird on record was nine years old when it was trapped and released during banding operations in June 2005.
During breeding season, Mountain Bluebirds seek out open areas with a mix of short grasses, shrubs, and trees, at elevations of up to 12,500 feet above sea level. They gravitate toward prairie and tundra edges, meadows, sagebrush flats, alpine hillsides, pastures, and recently burned or clearcut 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 Canada, and 31 percent in Mexico. They rate an 8 out of 20 on the Continental Concern Score. They are a U.S.-Canada Stewardship species, but they are not on the 2012 Watch List. These birds benefited from the westward spread of logging and grazing in the late nineteenth and early twentieth centuries, when the clearing of forest created open habitat for foraging. The subsequent waning of these industries, coupled with the deliberate suppression of wildfires, led to a dwindling of open acreage in the West and the decline of the species. More recently, as land-use practices have stabilized, so have Mountain Bluebird populations. Construction of nest boxes in suitable habitat has also provided a population boost. Populations are declining in areas where trees are too small to provide natural nesting cavities, and where forest and agricultural management practices have reduced the availability of suitable nest sites. Among birds that nest in cavities but can’t excavate them on their own, competition is high for nest sites. Mountain, Western, and more recently Eastern bluebirds compete for nest boxes where their ranges overlap. House Sparrows, European Starlings, and House Wrens also compete fiercely with bluebirds for nest cavities.
- Dunne, P. 2006. Pete Dunne’s essential field guide companion. Houghton Mifflin, Boston.
- Lukas, D. 2011. Sierra Nevada Birds: A Compact Field Guide Companion. Lukas Guides, Big Oak Flat, California.
- Power, H.W. and M.P. Lombardo. 1996. Mountain Bluebird (Sialia currucoides). In The Birds of North America Online, No. 222 (A. Poole, Ed.). Cornell Lab of Ornithology, Ithaca, New York.
- Partners in Flight. 2012. Species assessment database.
- USGS Northern Prairie Wildlife Research Center. 2013. Attracting bluebirds and other cavity nesting songbirds in North Dakota: establishing bluebird trails.
- USGS Patuxent Wildlife Research Center. 2013. Longevity records of North American Birds.
- USGS Patuxent Wildlife Research Center. 2012. North American Breeding Bird Survey 1966–2010 analysis.
Short- to medium-distance migrant. Mountain Bluebirds breed across western North America as far north as Alaska; they spend winters in as far south as central Mexico. Migrating flocks may consist of 20 to 200 birds.
Mountain Bluebirds take readily to nest boxes. If you live in suitably open habitat within their range, you can get plans for building Mountain Bluebird nest boxes, along with other tips, from our Nest Box Resource Center. Some retail outlets carry ready-made boxes. Nest boxes should be located away from buildings, areas of heavy pesticide use, and dense woods, ideally in open rural country with scattered clumps of trees or low shrubs. Mount boxes in pairs at least 100 yards apart, with 10 to 20 feet between boxes in a pair. Aggressive competitors can nest in the first box, leaving the second for bluebirds. The entrance hole should be about 1.75 inches in diameter, located about 6 inches above the floor of the box. It should face away from prevailing winds, and in an easterly direction to avoid overheating by afternoon sun. Ideally, there should be a place to perch within about 100 feet of the box, for when fledglings leave the nest.
Find This Bird
In the right places it can be a snap to find Mountain Bluebirds, as they are not shy of humans and live in fairly open country. They sit in the open on perches such as treetops, fence posts, and power lines. In summer in rural areas and ranches, particularly at higher elevations, you can often find them simply by driving rural roads and eyeballing such potential perches. In forested areas, look for them in large openings, particularly if there are aspen in the vicinity (aspen is a key cavity-providing tree in western montane forests). In winter, search for areas with berry-laden junipers and watch for flocks of birds feeding on those berries. Mountain Bluebirds often mix with Western Bluebirds, American Robins, and Cedar Waxwings when taking advantage of such fruity abundance.