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Western Bluebird Life History


Open WoodlandsWestern Bluebirds live in open woodlands and at the edges of woods. They are much less frequently seen in large meadows than either Eastern Bluebirds or Mountain Bluebirds. They live in evergreen and deciduous woods, particularly ponderosa pine but also pinyon pine-juniper, mixed conifers, and aspen stands. They thrive in disturbed areas such as burned forests or logged areas that still contain dead trees suitable for nesting and perching. During winter they inhabit pinyon-juniper woods, stands of mesquite, oak, or streamside woods, coastal chaparral, and desert.Back to top


InsectsDuring summer Western Bluebirds eat mainly insects; in winter they switch to eating mostly fruits and seeds, supplemented with insects. They typically catch ground-dwelling insects such as grasshoppers, caterpillars, beetles, ants, wasps, and pillbugs, as well as eating spiders and snails. They’ve been seen catching marine invertebrates on beaches. Winter foods include many kinds of berries, particularly elderberry, grapes, mistletoe, raspberries and blackberries, serviceberry, sumac, chokecherries, juniper, and poison oak. Back to top


Nest Placement

CavityA paired male and female search for nest sites together, inspecting cavities to see if they are suitable. Nests are placed in holes in trees (living or dead). Many kinds of trees are used, including pine, oak, aspen, willow, cottonwood, and sycamore, but they must contain a pre-existing cavity. Previous-years’ woodpecker nest holes are often used as well as natural tree holes, sometimes enlarged by other animals. Western Bluebirds readily take to nest boxes. They occasionally nest inside buildings or in the mud nests of swallows.

Nest Description

The females do almost all of the nest construction, gathering grasses, straw, pine needles, moss, other plant fibers, and fur from the ground and carrying it to the nest. She lines the nest cup with grasses, rootlets, feathers, horsehair, and sometimes bits of plastic. The nests are inside a cavity so they’re not as regularly shaped or tightly woven as typical cup nests. She may take 2 weeks to build her first nest of the season, but is much quicker when building a second nest of the year, completing it in under a week.

Nesting Facts

Clutch Size:2-8 eggs
Number of Broods:1-3 broods
Egg Length:0.8-2.4 in (2-6 cm)
Egg Width:0.7-0.9 in (1.8-2.4 cm)
Incubation Period:12-17 days
Nestling Period:18-25 days
Egg Description:Pale blue and unmarked, sometimes white.
Condition at Hatching:Bare, pink skin with sparse gray down, eyes closed.
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FlycatchingWestern Bluebirds tend to perch fairly low to the ground on prominent limbs, fence posts, and signs. They also tend to stay low to the ground when flying. They forage for insects by scanning the ground from a perch, then abruptly dropping to seize something they’ve spied. Outside the breeding season, Western Bluebirds are quite social, forming flocks up to about 100, sometimes with Mountain Bluebirds, American Robins, and Yellow-rumped Warblers. Their tree-cavity nest sites are a limited and valuable resource, and Western Bluebirds have to contend with competition from other bluebirds, swallows, nuthatches, some woodpeckers, House Wrens, and European Starlings. Western Bluebirds appear to be monogamous—one male pairs with one female to raise young each year. But experiments reveal that up to 45 percent of nests contain one or more young that are not the resident male’s offspring. Sometimes at nests, Western Bluebird pairs are joined by helpers who assist in raising the young. These are often adult males, but also include adult pairs and young birds.Back to top


Low Concern

Western Bluebirds are numerous, and populations held steady from 1966 to 2019, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 7.1 million and rates them 9 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Threats to this species primarily stem from the loss of habitat both from extensive logging and from growth of forests as a result of the suppression of natural fires; also development and grazing have reduced habitat availability. Even in appropriately wooded habitat, people may remove dead trees in an effort to clean up, limiting the places where bluebirds and other cavity nesters can find nest sites. Aggressive, non-native cavity nesters such as House Sparrows and European Starlings may take over many of the nest sites that Western Bluebirds might otherwise use.

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Guinan, J. A., P. A. Gowaty, and E. K. Eltzroth (2008). Western Bluebird (Sialia mexicana), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2019). The North American Breeding Bird Survey, Results and Analysis 1966–2019. Version 2.07.2019. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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