If we tried to design a bird to be popular with humans, it would be hard to invent anything more appealing than the Eastern Bluebird. It has beautiful colors, a soft, musical voice, and seemingly gentle behavior. It readily takes to birdhouses provided for it along the edges of yards and farms. What’s not to like? Its only serious competition might come from its two closest relatives. The three species of bluebirds that make up the genus Sialia form a very distinctive group, unique to North America.
The Eastern Bluebird is the most widespread of the three, with an odd and partly disjunct distribution. It’s a widespread breeder east of the Rockies in the U.S. and southern Canada, becoming scarce in southern Florida and southernmost Texas. But then it’s also resident in southeastern Arizona — the northern tip of a population that extends southward, mostly in the highlands, through Mexico, Guatemala, Honduras, and northern Nicaragua. A permanent resident in many areas, it withdraws in winter from the northernmost stretches of its range.
The Western Bluebird replaces the Eastern locally from the Rockies westward in the U.S. and southern Canada; the two species may overlap in the highlands of Mexico. Although it leaves some of its northernmost nesting areas in winter, it’s a permanent resident in many regions.
The Mountain Bluebird is the most migratory of the trio. Its breeding range extends from high mountains of the Southwest north to central Alaska, and most or all individuals leave their nesting areas in fall; some travel far south into Mexico or out onto the Great Plains. This is the bluebird that’s most likely to appear far out of range. It has been found in fall and winter in most of the eastern states and provinces. In summer, scattered individuals have been found far north of the Arctic Circle. On Victoria Island in Canada’s high Arctic, a pair of Mountain Bluebirds was found feeding young in a nest!
Eastern and Western Bluebirds have similar habitats and habits, favoring semi-open areas. Much of their diet, especially in summer, consists of insects. They often perch fairly low in farmland or woodland edges, such as on fence wires or low tree branches, flying down to pick up insects. Frequently, they hover for a few seconds before dropping to the ground. Mountain Bluebirds are often found in much more open situations, including grasslands with very few trees. They also take many insects from the ground, but they spend far more time hovering than the other two species.
Eastern Bluebirds almost always can be distinguished from the other two species by the field marks that I describe in the accompanying captions. There’s one tricky exception: Where the breeding ranges of Easterns and Mountains overlap on the northern Great Plains, they sometimes interbreed. Identifying a hybrid, especially a female, would require a very careful analysis of every field mark, and some individuals might have to be left unidentified.
What to look for
Size and shape. Size of a large sparrow but with more upright posture. Short, thin bill, round head, short neck, and fairly long wingtips.
Upperparts. Crown and back bright blue (male) to dull blue-gray (female), rarely with any brown.
Throat pattern. Chin usually whitish; lower throat rusty orange on male, whitish on female. Orange extends up onto side of neck on both.
Belly and undertail coverts. Always white, a helpful distinction from the other two bluebird species.
Wing structure. When perched, wingtips reach to base of tail. Primary extension shorter than tertials (about same length as tertials in Mountain Bluebird).
Female Eastern Bluebirds have essentially the same pattern as males, but they vary in brightness, some with vibrant colors and others more drab; the one in this portrait is about average. (Notes on Eastern Bluebirds in this column apply to the widespread forms north of the Mexican border; those in southeastern Arizona are duller overall.) They always show enough blue in the wings and tail to rule out confusion with unrelated birds, so the main challenge is separating them from female Western Bluebirds. The color of the throat can be hard to judge, as on this bird, but a wash of rusty orange extending up onto the side of the neck is diagnostic for Eastern Bluebird.
Juvenile bluebirds of all three species are heavily spotted on the breast at first, as is typical for members of the thrush family. On juvenile Eastern and Western Bluebirds, the back and scapulars also have prominent pale spots, but the juvenile Mountain Bluebird looks plainer above, with narrow white streaks. As with most songbirds, these bluebirds begin to molt out of juvenile plumage within a couple of months after leaving the nest, so there is only a brief time during which these youngsters are independent of their parents but still not in adult-like plumage. In those limited areas where multiple bluebird species are known to overlap in their breeding ranges, some molting juveniles may not be safely identified.
While the other two bluebird species are very similar in shape, the Mountain Bluebird usually looks a bit longer-billed. It has distinctly longer wingtips, with the primaries extending far beyond the tertials and at least halfway down the tail. The female Mountain Bluebird is very gray overall, with touches of blue only on the wings and tail. Its gray look and white eye-ring might suggest Townsend’s Solitaire, but that species has a buff wing patch and a longer tail with white outer tail feathers. In fresh plumage in late fall, some female Mountain Bluebirds have a wash of pale orange across the chest, and these might be confused with female Eastern Bluebirds.
No bird is truly unmistakable, but the adult male Mountain Bluebird, seen in good light, comes close. No other North American bird can match its overall sky-blue color. At times, though, Mountain and Western Bluebirds may range through the same juniper woods in winter. They tend to stick to their own flocks rather than mixing freely. When seen flying overhead, they can be distinguished by shape: the Mountain Bluebird has longer, more pointed wings, very noticeable with practice. The wing shape of this species is also reflected in its migrations — long-distance migrants tend to be longer-winged — and in its foraging behavior. Mountain Bluebirds spend substantial amounts of time hovering above the ground as they watch for insects below.
The female Western Bluebird is superficially very similar to the female Eastern in shape and overall color, although they seldom show as much blue as the brightest female Easterns. The pattern of the throat is a key difference. While many female Easterns are white on the throat, not orange, female Western Bluebirds have distinctly gray throats, showing almost no contrast against the gray face. The female Western often shows more of a brown wash across the back, but this is variable. The belly and undertail coverts of the Western are gray, not white as on the Eastern; but since this part of the bird is usually in shadow, this field mark is tricky to use.
Colorful male Western Bluebirds might seem like easy IDs, but they show a surprising amount of individual variation, which could lead to confusion. Most have a wash of rusty orange covering much of the upper back and scapulars, but on some, that color is limited (as on the bird in this photo) or entirely absent. Those with solid blue backs might suggest male Eastern Bluebirds. The extent of blue on the throat is variable also, but the lower throat is never orange as on the male Eastern. A further field mark for Western Bluebirds involves the belly and undertail coverts, which are gray, often with a strong wash of blue. On Easterns, these areas are white.
What’s the northern limit for Eastern Bluebirds in winter? Most field-guide maps draw the line near the southern Great Lakes and southern New England. But every year, there are scattered January and February records much farther north, up into central Maine, Quebec, northern Michigan, Minnesota, even North Dakota. Should the maps be redrawn? Not necessarily, because bluebirds are rare and irregular in those areas in winter. It would be misleading to color in the maps with solid color. It’s really hard to draw an accurate map for a species that occurs in very low densities at certain seasons.
Western Bluebirds are less variable in distribution, although they also withdraw in winter from some of their northern breeding areas. In the Southwest, they are common in some winters in lowland areas where mistletoe berries grow in good supply. In other winters, they are scarce in the valleys, concentrating instead in foothills and feeding on juniper berries.
Mountain Bluebirds are the most migratory species, vacating most of their breeding range in fall, but their winter distribution varies depending on food crops. In some seasons, they swarm through juniper woods with Westerns, but in others they move out to the most open desert or farmland, and they sometimes spread far eastward across the Great Plains. These nomads are worth watching for everywhere.
This article was first published in the September/October 2021 issue of BirdWatching magazine.