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Among the most readily watched and easily recognized of North American birds, Eastern Bluebirds are loved for their brilliant plumage, their tameness, and their predilection for nesting boxes. These small thrushes live from Ontario to Arizona, as well as in Mexico and Central America. Their accessible, open-landscape foraging and cavity-nesting habits facilitate observations by scientists and legions of hobbyists who claim that bluebird blue is “the most vivid” and bluebird warbles are “the most charming.”
For nesting, bluebirds depend on naturally occurring cavities or on cavities excavated or built by others in open habitats. Individuals hunt insects from perches over sparsely covered ground, migrate short distances or not at all, and have relatively simple songs and calls. Once seemingly threatened by nest-site competition from introduced species, Eastern Bluebird populations have been encouraged by enthusiasts, who provide hundreds of thousands of nesting boxes for their use.
Eastern Bluebirds comprise a species on a list of bird species, first noted as rare by Darwin (1871), in which the sex of juveniles—not just adults—is distinguishable by differences in plumage color and pattern, making them a “model” species for study of mating systems, sex ratios, and interactions between parents and offspring.
Eastern Bluebirds exhibit polygyny and polyandry as rare alternatives to their usual habit of monogamous pairings. Cooperative breeding is rare, in contrast to the similar Western Bluebird (Sialia mexicana). Despite their reputation as “sweet” and “models of pacific family life,” they are pugnacious in some situations, fighting most often with other species over access to nesting cavities. Not only males but females fight among themselves, and sometimes they kill each other over mates and nest sites. Extra-pair paternity is an important feature of bluebird life history, accounting for about 20% of nestlings in most populations.
The vocal behavior of this species is poorly known and would no doubt reward careful study. Few demographic summaries are available in reviewed literature, suggesting that banders who have detailed, systematic and controlled observations of individuals should be encouraged to publish.
Perhaps because Eastern Bluebirds are watched so often, a hallmark of the species’ biology, recognized even by the earliest observers, is a remarkable level of individual variation in morphology and behavior both within and between populations. Variable biology among individual Eastern Bluebirds guarantees that naturalist-scientists of the twenty-first century will continue explorations of the behavior and ecology of this species.