Bluebirds are cavity-nesting songbirds belonging to the genus Sialia and the thrush family. They are instantly identified by their vibrant cobalt plumage. This striking blue color is not due to pigment but to the unique structure of their feathers, which scatter light to reflect the blue wavelength. When people refer to “the bluebird,” they are actually speaking of three distinct species found across North America.
Shared Physical Traits and Size
All three Sialia species share a similar compact, robust body shape. They are considered medium-sized thrushes, generally smaller than an American Robin but slightly larger than a sparrow. An average bluebird measures approximately 6.3 to 8.3 inches (16 to 21 centimeters) in length.
Bluebirds have a stocky body and a relatively large, rounded head. They possess short legs and a thin, straight, black bill suited for catching insects. When perched, they typically sit upright on an elevated position, scanning the ground for prey. Their weight usually falls between 0.95 and 1.20 ounces (27 to 34 grams).
The Three Distinct Species and Their Coloration
The three bluebird species—Eastern, Western, and Mountain—are differentiated primarily by the specific patterns of their brilliant coloration. Male Eastern Bluebirds (Sialia sialis) display vivid royal blue on their head, back, and wings. This deep blue is sharply contrasted by a rusty or reddish-brown chest and throat, which gives way to a white lower belly. Females exhibit the same pattern, but their colors are much duller, featuring a grayish-blue back and muted orange-brown on the breast.
The male Western Bluebird (Sialia mexicana) presents a different pattern of blue and chestnut. They have a bright blue head and back, but the reddish-brown chest color often extends up to form a distinct rusty patch across the upper back. A key field mark is that the male Western Bluebird’s throat is blue, unlike the Eastern species’ rusty throat. Female Western Bluebirds are generally grayish-brown on the head and breast, retaining some blue coloration on their wings and tail.
The Mountain Bluebird (Sialia currucoides) is the most uniformly colored of the three species, lacking rusty or chestnut markings entirely. Adult males are almost entirely a brilliant turquoise or sky-blue, appearing paler below and darker above. The color is uninterrupted across the body, with only a pale white lower abdomen. Females are predominantly a plain gray-brown, with subtle washes of pale blue visible on the wings and tail.
Distinguishing Bluebirds From Other Blue Birds
Bluebirds are often confused with other North American species that display blue feathers, requiring attention to size and structure for correct identification. A common look-alike is the Blue Jay, but it is substantially larger than any bluebird and possesses a prominent crest. Blue Jays also have complex patterns of black barring and white patches on their wings and tail, which are absent on the more solidly colored bluebirds.
Another small blue species is the male Indigo Bunting, which is noticeably smaller than a bluebird. The bunting is entirely a deep, electric blue without any rusty or brown patches, and it has a small, conical bill adapted for eating seeds. Conversely, the Blue Grosbeak is similar in size to a bluebird, but it is immediately distinguished by its heavy, thick, seed-crushing bill and two distinct rust-colored bars on its wings. Bluebirds, in contrast, possess the slender, pointed bill necessary for their insect-rich diet.