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.Back to top
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.Back to top
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.
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.
|Clutch Size:||4-8 eggs|
|Number of Broods:||1-2 broods|
|Egg Length:||0.8-1.0 in (1.9-2.5 cm)|
|Egg Width:||0.6-0.7 in (1.6-1.7 cm)|
|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.|
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. Back to top
Mountain Bluebirds are fairly common, but populations declined by about 24% between 1966 and 2015, according to the North American Breeding Bird Survey. Partners in Flight estimates the global breeding population of 4.6 million, with 80% spending some part of the year in the U.S., 20% breeding in Canada, and 31% wintering in Mexico. The species rates a 12 out of 20 on the Continental Concern Score. Mountain Bluebird is not on the 2016 State of North America's Birds' Watch List, but it is a U.S.-Canada Stewardship species. These bluebirds 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. Back to top
Mountain Bluebirds take readily to nest boxes. If you live in suitably open habitat within their range, consider putting up a nest box to attract a breeding pair. Make sure you put it up well before breeding season. 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. Attach a guard to keep predators from raiding eggs and young. Find out more about nest boxes on All About Birdhouses. Some retail outlets carry ready-made boxes, or build your own: you'll find plans for building a nest box of the appropriate size for Mountain Bluebirds on our NestWatch site.Back to top
Dunne, P. (2006). Pete Dunne's essential field guide companion. Houghton Mifflin Harcourt, New York, USA.
Lutmerding, J. A. and A. S. Love. (2019). Longevity records of North American birds. Version 1019 Patuxent Wildlife Research Center, Bird Banding Laboratory 2019.
Partners in Flight (2017). Avian Conservation Assessment Database. 2017.
Power, H. W., and M. P. Lombardo (1996). Mountain Bluebird (Sialia currucoides), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.
Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.
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