The term “bluebird” most commonly refers to the three species native to North America, all belonging to the genus Sialia. While bluebirds were once in serious decline, their populations have largely rebounded today due to decades of conservation efforts. Their status has shifted from a species of concern to a widespread success story, though their continued well-being still relies on human intervention and monitoring.
Defining the Species
The rarity of a bluebird depends on which of the three distinct species is being discussed, as each occupies a different geographic range. The Eastern Bluebird (Sialia sialis) is the most familiar, characterized by a rusty-red breast and throat contrasting with its deep blue head, back, and wings. This species is found east of the Rocky Mountains, spanning from southern Canada to Central America.
In the western half of the continent, two other species are found, often in overlapping territories. The male Western Bluebird (Sialia mexicana) shares the rusty coloration but features a distinctive blue throat and a rusty patch extending onto the upper back and sides. The Mountain Bluebird (Sialia currucoides) is visually the most striking; the male is almost entirely a brilliant, sky-blue color across its head, back, and breast, with only a small patch of white near the belly.
Current Population Status
Bluebirds faced a severe crisis in the mid-20th century, with populations plummeting due to reduced nesting sites and food sources. Bluebirds are secondary cavity nesters, meaning they cannot excavate their own nests and rely on existing holes in dead trees or wooden posts. Habitat loss from clearing orchards and fields removed many snags and fence posts that provided these necessary cavities.
Compounding this was the introduction of aggressive, non-native species—the European Starling and the House Sparrow—which competed fiercely for limited nesting cavities. Furthermore, the widespread use of pesticides reduced the insect populations that form the bulk of the bluebird’s summer diet.
A grassroots conservation movement successfully reversed this trend through the widespread deployment of specially designed nest boxes. These small wooden structures, often installed along “bluebird trails,” were constructed with entrance holes sized to exclude larger European Starlings while admitting the bluebirds. This effort provided a safe and abundant alternative to natural cavities. The North American Breeding Bird Survey confirms that the population of the Eastern Bluebird has increased significantly since the late 1960s. None of the three species are currently listed as federally threatened or endangered, confirming that the bluebird is common across its range today.
The Science Behind Blue Feathers
The vivid blue coloration does not come from a pigment, unlike the red in a cardinal or the yellow in a goldfinch. In fact, no true blue pigment exists in any bird. The color is structural, created by the way light interacts with the microscopic physical structure of the feathers.
The barbules contain a matrix of tiny air pockets and keratin protein structures. When sunlight hits this structure, the microscopic components scatter the shorter blue wavelengths of light much more effectively than the longer red and yellow wavelengths. This preferential scattering causes the observer to perceive the color as blue.
The underlying layer contains dark melanin granules, which absorb the unscattered longer wavelengths of light. This absorption prevents other colors from interfering, intensifying the resulting blue hue. This structural mechanism explains why the blue color can appear to shift slightly in different lighting conditions or viewing angles, as the light reflection changes.
Habitat and Geographic Range
Bluebirds favor open landscapes with scattered trees, a habitat type often described as edge habitat. They are not found in dense, unbroken forests. Their preferred environments include pastures, orchards, golf courses, and open woodlands where the ground cover is sparse.
This preference is tied to their foraging behavior; bluebirds are adept at hunting insects from a low perch, such as a fence post or telephone wire, before swooping down to capture prey. The Mountain Bluebird is an exception, often found at higher elevations and known for hovering in the air to spot insects, a behavior less common in the other two species. While their summer diet is dominated by insects, they switch to consuming small fruits and berries, such as mistletoe and hackberry, to sustain themselves through the winter months.