Baseline hormone levels are linked to reproductive success but not parental care behaviors.

Consistent behavioral differences among individuals, or personalities, have been hypothesized to arise as a result of consistent individual differences in hormone levels. Individual variation in baseline hormone levels or hormonal similarity within a breeding pair may be related to reproductive success, as suggested by the corticosterone-fitness hypothesis and the hormonal similarity hypothesis, respectively. In a population of Eastern bluebirds (Sialia sialis) with repeatable behavioral expression and coordination of behavior within pairs, we tested if baseline androgen and corticosterone levels are related to behavioral expression, if coordination in behavior within pairs is facilitated by hormonal coordination, and if baseline hormone levels are related to fledging success at the individual or pair level.

Structural coloration signals condition, parental investment, and circulating hormone levels in Eastern bluebirds (Sialia sialis).

Many of the brilliant plumage coloration displays of birds function as signals to conspecifics. One species in which the function of plumage ornaments has been assessed is the Eastern bluebird (Sialia sialis). Studies of a population breeding in Alabama (USA) have established that plumage ornaments signal quality, parental investment, and competitive ability in both sexes.