Abstract
Male eastern bluebirds (Sialia sialis) have feathers with either structurally based blue coloration or melanin-based chestnut coloration, and they hold territories during the breeding season that they defend vigorously. We tested whether the melanin pigmentation or structural coloration of feathers serve as signals during intrasexual aggressive encounters by placing color-modified stuffed bluebirds in male territories. We recorded the time to attack and the intensity of attacks on each model, and we then compared the response to color-enhanced versus color-reduced models. Male bluebirds attacked models with brighter and more chromatic blue coloration significantly more often than they attacked models with darker and less chromatic blue coloration. In contrast, the darkness of the chestnut breast coloration did not have a significant effect on the rate at which models were attacked. We conclude that territorial male bluebirds perceive intruding males with brighter blue coloration as a greater threat than males with drabber blue coloration, presumably because blue coloration is a signal of fighting ability. In contrast, the chestnut coloration of breast feathers appears to be a signal of gender and sexual maturity and not a graded signal of social status.
Highlights
Sexual selection on a trait can arise either through female mate choice for the trait or through advantages that result from the trait in male-male interactions (Darwin, 1871; Andersson, 1994; Hill, 2014)
Among the 11 trials in which the model receiving the most aggression differed from the model receiving the first aggression, the responding male switched the focus of attack from the drab to the bright model in six trials (55%) and from the bright to the drab model in five trials (45%), indicating that the attacks of these males were not focused on a particular model (Fisher’s exact test, P = 0.99)
Previous studies showed that blue coloration but not chestnut coloration is associated with individual quality in eastern bluebirds
Summary
Sexual selection on a trait can arise either through female mate choice for the trait or through advantages that result from the trait in male-male interactions (Darwin, 1871; Andersson, 1994; Hill, 2014). There is an abundance of evidence from studies of diverse taxa that visual displays can serve as signals of male fighting ability and that these signals can mediate contests between males (Andersson, 1994; Berglund et al, 1996; Tibbetts, in press). Within the context of competition among all males in a population, both dominant and subordinate males benefit from honest signaling of status (Rohwer, 1982; Senar, 2006). Dominant males avoid wasting time and energy engaging in contests with lower-status males, and subordinate males benefit by avoiding dangerous interactions with dominant individuals (Rohwer, 1975; Lyon and Montgomerie, 1986; Hawkins et al, 2012). Status signals can be used by birds as honest indicators of a rival’s age, rank, or fighting ability (Fugle et al, 1984; Jarvi and Bakken, 1984; Senar, 2006)
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