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Eastern Bluebird
Sialia sialis
– Family
Authors: Gowaty, Patricia Adair, and Jonathan H. Plissner
Revisors: Gowaty, Patricia Adair

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Figure 1. Distribution of the Eastern Bluebird.
Adult male Eastern Bluebird, NJ, 18 April.
Adult female Eastern Bluebird, Hadley, Hampshire Co., MA, 10 December.

Eastern Bluebirds have gorgeous plumage, nest in open, often disturbed, human-modified habitats, and, as secondary cavity nesters, readily take to artificial nesting cavities. In the breeding season, they hunt insects from perches over sparsely covered ground or low grass, and in the winter join mixed species foraging flocks in woods and hedgerows, as well as near places suitable for nesting.

Because this species often uses artificial nesting cavities and forages in the open, it is a bird that humans frequently watch and study. Over the last 40 years Eastern Bluebirds have become a model species for field study of fundamental questions in ecology, evolution, behavioral ecology, and hormones and behavior, as well as diseases and health, life history and morphology and ultrastructure of coloration. Bluebirds will no doubt be a species of choice in the vanguard of avian species for the study of epigenetics (gene regulation in response to variable ecological and social conditions).

Along with scientific attention has come the North American Bluebird Society (NABS), a consortium of lay observers who continue to construct bluebird trails (transects with maintained bluebird boxes) and to foster the conservation of bluebirds and other cavity nesters in North America. NABS observers contribute to our basic understanding of bluebird ecology, demography, and behavior. The Cornell Lab of Ornithology, along with a network of lay observers, is reporting on comparative geographic gradients in life history variables of Eastern Bluebirds to elucidate questions about the adaptive significance of geographic trends, an effort that is significantly increasing our understanding of reproductive patterns and the evolution of life histories, as well as engaging citizens in basic science.

The differences in the plumages of female and male Eastern Bluebirds have  contributed to the popularity of this species as ordinary observers are able to see what individuals of different sexes do, an opportunity few backyard birds of North America offer. The distinctive sexual differences of eastern bluebirds make them easier to relate to than monochromatic passerines. The fact that the sexes are distinguishable has made the species a model for the study of behavioral ecology and evolution: Studies of mating behavior; mating system variation; adult, juvenile, and nestling sex ratios; parent-offspring interactions; dispersal patterns; and migratory patterns. More recently, attention to the adaptive significance of within- and between sex variation in plumage coloration (and egg coloration) is informing answers to questions about naturally and sexually selected visual signaling as well as answers to basic questions of how bluebirds make “bluebird blue”. Notably bluebirds are on the list of bird species, first reported as rare by Darwin (1871), in which the sex of juveniles—not just adults—is distinguishable by differences in plumage color and pattern, the significance of which remains poorly understood but which is now receiving more detailed experimental attention along with systematic and objective characterization of individual variation in coloration. Bluebirds are sexually distinctive to our eyes, but even more sexually distinctive to each other because, unlike humans, they are able to see in the ultra-violet, and we know from recent studies that bluebird males reflect more strongly in the UV than do females.

Though Eastern Bluebird songs and calls may appear simple, recent study revealed that, among adult males in the same area, calls and songs vary greatly  even within the same individual. Bluebirds have a relatively large repertoire of soft and loud songs and calls.

Eastern Bluebirds are “classically” socially monogamous. Rarely, they also nest in socially polygynous and socially polyandrous associations and, just as rarely, as cooperative breeders in which adult offspring “help” their parents raise broods. They were the first socially monogamous bird species to be discovered to have females that mate with more than one male, resulting in extra-pair paternity, which means that males often care for offspring not genetically their own -- emphasizing a conundrum for evolutionary biologists who predicted that males should defend their genetic paternity. Conspecific nest parasitism, in which females lay eggs in other females’ nests, also occurs, so that even females are sometimes caring for offspring not theirs.

Despite having once been called the “North American example of pacific family life,” and despite their reputation for being “sweet”, they are in fact pugnacious defenders of territories, mates, and nests against interspecific and conspecific competitors. They contend with other species and with each other over access to nesting cavities. Not only males but also females fight among themselves, and sometimes females wound and kill each other over access to nesting sites. Nevertheless, male aggression against females is ecologically contingent, rare in most situations.

Fledgling behavior -- behavior of birds from the time of nest leaving until the post-juvenile molt -- is relatively poorly known. Of interest are questions about sibling interactions by sex and age during this period, juvenile flock formation dynamics, and hallmarks of the development of foraging competence, and behavior associated with emigration.

Natal and breeding dispersal patterns in S. Carolina and Georgia are well described. It is now known how far individuals typically move. What remains entirely unknown is what happens along the way from one site to another during dispersal events.

Few demographic summaries of individually identifiable birds are available in the reviewed literature, suggesting that banders who have detailed, systematic and controlled observations of individuals should be encouraged to publish. Fitness is a relative concept and thus studies of adaptive significance should be in the context of local demography, including variation in lifespan. Predictions of new models of how stochastic demography affect adaptive mating decisions (e.g., (Gowaty and Hubbell 2009) would be ideally tested in bluebirds.

Experimental studies of female mate preferences failed to identify the basis for female choice of male bluebirds, suggesting that something may be left out of general models of pairing and mating. Migratory behavior, as distinct from dispersal behavior, also remains poorly studied. As the technologies for marking and remote sensing of individuals are becoming more refined and affordable, Eastern Bluebirds will be good candidates for range-wide study of the timing and extent of migratory movements as a function of environmental inducers.

An emerging hallmark of bluebird biology, recognized even by the earliest observers, is a remarkable level of individual variation in morphology and behavior both within and between populations, as well as data indicating that bluebirds make flexible and adaptive decisions to use or not use nest boxes of different qualities or history -- showing that ecological and social contingencies matter to what bluebirds do. Given such known variability among individuals, a comprehensive view of bluebird ecology, demography, and behavior demands study of bluebirds in natural, perhaps ancestral habitats such as longleaf pine forests of the se. USA and in understudied regions in their range (e.g., Mexico, Guatemala, and other areas in Central America).