in 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.