- 6.3–7.5 in
- 0.8–1.1 oz
- Larger than a sparrow; smaller than an American Robin.
- Merleblue de l'Ouest (French)
- Azulejo garganta azul (Spanish)
- Western Bluebirds are among the birds that nest in cavities—holes in trees or nest boxes. But look at their bills—they’re not equipped to dig out their own holes. They rely on woodpeckers or other processes to make their nest sites for them. This is one reason why dead trees are a valuable commodity in many habitats.
- Occasionally Western Bluebirds have helpers at the nest. Most of the extra birds attending nests are helping their presumed parents, some after their own nests have failed. Interestingly, studies show that many nests include young that were not fathered by the resident male.
- Genetic studies showed that 45% of nests had young that were not fathered by the defending male, and that 19% of all the young were fathered outside the pair bond.
- Western Bluebirds have a gentle look, but territory battles can get heated. Rival males may grab each other’s legs, tumble to the ground, and then pin their opponent on the ground, stand over him, and jab at him with his bill.
- A Western Bluebird weighs about an ounce. It needs about 15 calories (technically, kilocalories) per day, or 23 calories if raising young.
- The oldest known Western Bluebird was a male, and at least 8 years, 8 months old when he was found in California.
Western 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.
During 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.
- Clutch Size
- 2–8 eggs
- Number of Broods
- 1-3 broods
- Egg Length
- 0.8–2.4 in
- Egg Width
- 0.7–0.9 in
- 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.
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.
A 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.
Western 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.
Western Bluebirds are numerous and populations were stable from 1966 to 2014, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 6.7 million with 67% spending some part of the year in the U.S., 52% in Mexico, and 1% breeding in Canada. They rate a 9 out of 20 on the Continental Concern Score and are not on the 2014 State of the Birds Watch List. 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; this limits 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.
- Guinan, J. A., P. A. Gowaty, and E. K. Eltzroth. 2000. Western Bluebird (Sialia mexicana). In The Birds of North America, No. 510 (A. Poole and F. Gill, eds.). The Birds of North America Online, Ithaca, New York.
- North American Bird Conservation Initiative, U.S. Committee. 2014. State of the Birds 2014 Report. U.S. Department of Interior, Washington, DC.
Partners in Flight. 2012. Species assessment database.
- USGS Patuxent Wildlife Research Center. 2015. Longevity Records of North American Birds.
USGS Patuxent Wildlife Research Center. 2014. North American Breeding Bird Survey 1966–2014 Analysis.
Resident to medium-distance migrant. Some Western Bluebirds even as far north as British Columbia, Canada, stay there year-round. Bluebirds in mountainous areas may simply move down the slope to warmer elevations for winter. Others across the interior West depart their breeding grounds, and wintering birds begin to be seen in Arizona, west Texas, and northern Mexico by October and November. Unlike many songbirds, Western Bluebirds often migrate during the day.
Western Bluebirds are mainly insectivorous in the summer and they can be attracted to feeders if you offer mealworms. (More on mealworms at our Attract Birds section).
You can also invite bluebirds to a partially wooded yard by putting up nest boxes. Make sure you put up a nest box well before breeding season. Attach a guard to keep predators from raiding eggs and young. Find out more about nest boxes on our Attract Birds pages. You'll find plans for building a nest box of the appropriate size on our All About Birdhouses site.
Find This Bird
Look for Western Bluebirds on low perches in woodlands and woodland edges. Also scan for them sitting atop nest boxes or fenceposts in summer. Their habit of dropping suddenly to the ground after insects can be recognizable even out of the corner of your eye. Their quiet, inquisitive call notes are easy to overlook, but distinctive once learned.