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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 a seemingly gentle demeanor. It easily adapts to the nesting boxes reserved for it along yards and farms. What’s not to like? His only serious competition might come from his two closest relatives. The three species of bluebirds that make up the genus Sialia form a very distinctive group, unique in North America.
The Eastern Bluebird is the most widespread of the three, with an odd and partially disjunct distribution. It is a widespread breeder east of the Rockies in the United States and southern Canada, becoming rare in southern Florida and extreme southern Texas. But it also resides in southeastern Arizona – the northern tip of a population that extends south, mostly into the highlands, through Mexico, Guatemala, Honduras and northern Nicaragua. . A permanent resident in many areas, it retreats in winter from the northernmost parts of its range.
The Western Bluebird locally replaces the Eastern Bluebird from the Rocky Mountains west into the United States and southern Canada; the two species may overlap in the highlands of Mexico. Although it leaves some of its northernmost breeding grounds in winter, it is a permanent resident in many areas.
The Mountain Bluebird is the most migratory of the trio. Its breeding range extends from the high mountains in the north of the southwest to central Alaska, and most or all individuals leave their nesting grounds in the fall; some travel far south to Mexico or the Great Plains. It is the blue bird that is most likely to appear out of range. It has been found in fall and winter in most eastern states and provinces. In summer, scattered individuals have been found far north of the Arctic Circle. On Victoria Island in the Canadian High Arctic, a pair of Mountain Bluebirds have been found feeding their 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 quite low in farmland or wood edges, such as on fence wires or low tree branches, flying to pick up insects. Often they hover for a few seconds before falling back to the ground. The Mountain Bluebird is often found in much more open situations, including grasslands with very few trees. They also take many insects from the ground, but spend much more time hovering than the other two species.
The Eastern Bluebird is almost always distinguished from the other two species by the terrain markings I describe in the accompanying captions. There is one tricky exception: where the breeding ranges of the Easterns and Mountains overlap in the northern Great Plains, they sometimes intersect. Identifying a hybrid, especially a female, would require very careful analysis of each ground mark, and some individuals might have to remain unidentified.
What to look for
The size and the shape. Size of a large sparrow but with a more upright posture. Short, thin beak, round head, short neck and fairly long wingtips.
Upper parts. Crown and back bright blue (male) to dull blue-gray (female), rarely with brown.
Groove pattern. Chin generally whitish; lower throat rusty orange in males, whitish in females. Orange extends down the side of the neck on both.
Belly and undertail coverts. Always white, a useful distinction from the other two blue bird species.
Wing structure. When perched, the wingtips reach the base of the tail. Primary extension shorter than tertials (about the same length as tertials in the mountain bluebird).
Eastern bluebird, adult female. April in Montgomery County, Texas. Photo by Brian E. Small
Female Eastern Bluebirds have essentially the same pattern as the males, but vary in brightness, some with bright colors and others more dull. that of this portrait is average. (Notes on Eastern Bluebirds in this column apply to forms prevalent north of the Mexican border; those in southeastern Arizona are duller overall.) They still show sufficient blue in the wings and tail to exclude confusion with unrelated birds, so the main challenge separates them from female Western Bluebirds. Throat color can be difficult to judge, as on this bird, but a wash of rusty orange extending down the side of the neck is diagnostic of the Eastern Bluebird.
Eastern bluebird, juvenile male. June in New Haven County, Connecticut. Photo by Brian E. Small
Juvenile bluebirds of all three species are heavily mottled on the breast at first, as is typical for members of the thrush family. In young eastern and western bluebirds, the back and scapulars also have prominent pale spots, but the juvenile mountain bluebird appears paler above, with narrow white streaks. As with most songbirds, these bluebirds begin to moult from juvenile plumage a few months after leaving the nest, so there is only a brief time during which these young are independent of their parents but still not in adult plumage. In limited areas where multiple bluebird species overlap in their breeding ranges, some molting juveniles may not be safely identified.
Mountain bluebird, adult female. December in Socorro County, New Mexico. Photo by Brian E. Small
While the other two bluebird species are very similar in shape, the mountain bluebird generally has a somewhat longer beak. It has noticeably longer wingtips, with the primaries extending well beyond the tertials and at least halfway to the tail. The female Mountain Bluebird is very gray overall, with blue accents only on the wings and tail. Its gray appearance and white eyering might suggest Townsend’s Solitaire, but this species has a buff wing patch and a longer tail with white outer feathers. In fresh plumage in late fall, some female mountain bluebirds have a pale orange tint to their breasts, and these could be confused with female mountain bluebirds.
Mountain bluebird, adult male. June in Kamloops, British Columbia. Photo by Brian E. Small
Neither bird is truly unmistakable, but the adult male Mountain Bluebird, seen in good light, comes close. No other North American bird can match its sky blue color. Occasionally, however, the Mountain Bluebird and Western Bluebird may pass through the same juniper woods in winter. They tend to stick to their own herds rather than mingling freely. When they fly above our heads, they are distinguished by their shape: the mountain bluebird has longer and more pointed wings, very visible with training. The wing shape of this species is also reflected in its migrations – long-distance migrants tend to have longer wings – and in its foraging behavior. Mountain bluebirds spend a lot of time hovering above the ground looking for insects below.
Western bluebird, adult female. November in Santa Barbara County, California. Photo by Brian E. Small
The female Western Bluebird is superficially very similar to the female Eastern in shape and general color, though she rarely shows as much blue as the brighter eastern females. The throat pattern is a key difference. While many eastern females have white, not orange, throats, female Western Bluebirds have distinctly gray throats, showing almost no contrast to the gray face. The Western female often shows more of a brown wash on the back, but this is variable. The belly and undertail-coverts of the West are gray, not white as in the East; but as this part of the bird is usually in shadow, this field mark is tricky to use.
Western bluebird, adult male. April in Santa Barbara County, California. Photo by Brian E. Small
Colored male Western Bluebirds may seem like easy identifications, but they show a surprising amount of individual variation, which could be confusing. Most have a wash of rusty orange covering much of the upper back and scapulars, but on some this color is limited (as in the bird in this photo) or absent altogether. Those with solid blue backs might suggest male Eastern Bluebirds. The extent of blue on the throat is also variable, but the lower throat is never orange as in the eastern male. Another terrain mark for the Western Bluebird is the belly and undertail coverts, which are grey, often with a strong blue tint. On Easterns, these areas are white.
Complex migrations
What is the northern limit of the Eastern Bluebird 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, as far north as central Maine, Quebec, northern Michigan, Minnesota, and even North Dakota. Should the maps be redrawn? Not necessarily, as bluebirds are rare and spotty in these areas in winter. It would be misleading to color the cards with a solid color. It is really difficult to draw an accurate map for a species that occurs in very low densities in certain seasons.
The Western Bluebird has a less variable distribution, although it also retreats in winter from some of its northern breeding grounds. In the southwest, they are common in some winters in lowland areas where mistletoe berries grow in sufficient quantity. In other winters, they are rare in the valleys, concentrating instead in the foothills and feeding on juniper berries.
The mountain bluebird is the most migratory species, leaving most of its breeding range in the fall, but its winter distribution varies among food crops. At certain seasons they swarm through the juniper woods with Westerns, but at others they move into the more open desert or farmland, and they sometimes spread far east across the Great Plains. These nomads are worth watching everywhere.
This article first appeared in the September/October 2021 issue of BirdWatching magazine.
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Listen to the calls of the Eastern Bluebird here.
