Effect of prenatal and natal administration of testosterone on production of structurally based plumage coloration

Testosterone has been implicated as a developmental mechanism involved in the organization and expression of sexually dimorphic traits, such as plumage coloration, in birds. Although research findings relating testosterone levels to plumage expression is equivocal, few studies have investigated how testosterone may influence the expression of structurally based plumage coloration. Here, we use experimental and correlational evidence to test the hypothesis that testosterone influences the development and maintenance of structurally based plumage coloration in a wild-breeding population of eastern bluebirds (Sialia sialis). First, we experimentally manipulated yolk testosterone and measured the effect on the development of plumage coloration of nestlings. Second, we implanted juvenile bluebirds with testosterone and measured the effect on nestling growth, body condition, and plumage coloration of nestlings. Third, we measured covariation between circulating testosterone and plumage coloration of breeding males. Yolk testosterone injections had no significant effect on nestling plumage coloration. Testosterone implantation, however, caused a reduction in plumage brightness, elevated corticosterone, and slower growth in nestlings. Finally, in breeding adult males we found no significant relationship between structural coloration and testosterone; however, males with higher testosterone levels exhibited duller chestnut (melanin-based) plumage. Our observations lead us to reject the hypothesis that testosterone increases structural plumage coloration in male eastern bluebirds.