North America is home to three distinct species of bluebirds from the thrush family: the Eastern Bluebird (Sialia sialis), the Western Bluebird (Sialia mexicana), and the Mountain Bluebird (Sialia currucoides). While they share a genus and vibrant blue coloration, each species occupies a unique geographic range and possesses distinguishing physical characteristics. Understanding the total population requires examining the individual counts and distributions of these three relatives.
Identifying the Three North American Species
The three bluebird species are identified by their plumage patterns and the regions they inhabit. The Eastern Bluebird is the most widespread, covering the area east of the Rocky Mountains, and the male is recognized by bright blue upperparts contrasted with a rusty-orange throat and chest that fades to a white abdomen. The Western Bluebird, found from the Rocky Mountains to the Pacific Coast, also displays an orange breast, but the male’s blue is often deeper, and the orange extends onto the upper back. The Mountain Bluebird prefers high-elevation, open country in the West. The male is visually distinct, displaying a nearly uniform, brilliant sky-blue across its entire body, lacking the prominent orange markings of the other two species.
Current Population Estimates and Geographic Distribution
Estimates from conservation organizations like Partners in Flight provide a snapshot of the number of breeding bluebirds across the continent. The total population of all three species combined is approximately 35.7 million breeding individuals. This figure is dominated by the Eastern Bluebird, which is estimated to have a global breeding population of 23 million. This population is broadly distributed across the open woodlands, fields, and orchards spanning from southern Canada down to Central America and west to the Great Plains.
The Western Bluebird population is estimated at 7.1 million individuals, making it the second most common species. Its distribution is confined to the western third of North America, where it favors semi-open woodlands and forest edges. The Mountain Bluebird has the smallest population estimate, with about 5.6 million breeding individuals. This species inhabits the high-elevation grasslands and mountain meadows of the West, including the Rocky Mountains, with a range that extends into Alaska and down to central Mexico during the non-breeding season.
Monitoring Population Trends and Conservation Success
The current healthy population numbers represent a significant recovery from historical lows experienced in the mid-20th century. Between 1920 and 1970, bluebird populations, particularly the Eastern Bluebird, plummeted due to the widespread removal of dead trees (snags), which provide natural nesting cavities. A primary cause of the decline was intense competition for remaining nesting sites from the aggressive, non-native European Starling and House Sparrow. At its lowest point, the Eastern Bluebird population fell by as much as 90% in some regions.
The rebound began in the 1960s and 1970s with the rise of widespread conservation efforts and citizen science initiatives. The North American Bluebird Society, formed in 1978, advocated for the installation of specialized nest boxes along “bluebird trails.” These boxes were designed with entrance holes sized to admit bluebirds but exclude the larger European Starling, alleviating competition. This intervention proved highly effective, and the Eastern Bluebird population has shown a strong, statistically significant increase since 1966, according to the North American Breeding Bird Survey (BBS) data.
Long-term monitoring through methods like the BBS and data compilation by Partners in Flight continues to track these populations. Western Bluebird numbers have remained steady over the past several decades, while the trend for Mountain Bluebirds shows some regional declines. While all three species are currently classified as being of low conservation concern, continued tracking is necessary to inform habitat management and ensure the success of these birds endures.