Intraspecific signaling functions of juvenal plumage

Abstract

The function and evolution of avian plumage coloration has been the subject of many studies over the past decade, but virtually all of this research has focused on the plumages of sexually mature individuals. The color and pattern of juvenal plumage, which is worn only for the first few months of life by altricial songbirds, remains essentially unstudied. To examine how juvenal plumage functions in intraspecific contexts, we developed a three-pronged approach designed to uncover the evolutionary origins and current adaptive functions of several juvenal plumage traits. First, we developed the idea that distinctive juvenile appearance may be a signal of sexual immaturity, serving to reduce aggression from conspecific adults. We used a comparative approach to test this hypothesis in the thrushes (Family Turdidae) and found that distinctive juvenile appearance was significantly correlated with increased risk of conspecific aggression. The observed associations are consistent with our hypothesis that the benefits of signaling sexual immaturity have driven the evolution of distinctive juvenal plumage. Second, we tested the applicability of the comparative study results by employing a field study of eastern bluebirds, Sialia sialis. We tested the hypothesis that spotty plumage of eastern bluebird juveniles’ signals age, thereby decreasing aggression from territorial adults, by measuring the aggressive responses of adult bluebird males to different combinations of simultaneously presented taxidermic mounts. We found that territorial males do not recognize juvenile-specific plumage, but avoid attacking intruders that lack adult plumage characters. Third, we tested the idea that plumage color may serve as a signal of quality in fledgling eastern bluebirds and that parental feeding decisions are influenced by differences between offspring with respect to plumage color. Fledglings were presented in pairs and parental investment was scored as a percentage of feeding attempts to each individual. Our results support the hypothesis that feather coloration of male bluebird fledglings affects the care they receive from parents. Eastern bluebird parents appear to use juvenal plumage coloration as a signal to assess the relative quality of offspring after fledging and to adjust parental investment in a manner that maximizes their reproductive success.