Breeding Adaptations in the Eastern Bluebird

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Abstract
Adaptations in the breeding of eastern bluebirds [Sialia sialis] were studied in southeastern Michigan [USA] during 1968-1976. No temproal differences were found in the arrival pattern of yearlings compared with adults, but yearlings (especially males) began nesting later, sometimes ceased breeding earlier, and reared fewer young per season than adults. Otherwise, nesting parameters (clutch size, nesting success, brood size and fledgling survival) were not significantly different for the 2 age groups. Females not rearing a brood earlier in the same season laid larger clutches than those having a previous brood. Nesting success was equal in females that did and those that did not have an earlier brood. Males that did not have an earlier brood had greater success although they appeared to be restricted to poorer habitats. The reduced clutch size of females already successful may result from impaired physiological condition, a consequence of rearing an earlier brood. Tolerance for reduced physiological condition for the sake of reproduction would seem desirable in secondary cavity-nesters, although it does not appear to be as great in male bluebirds as in females.

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