UV-blue structural coloration and competition for nestboxes in male eastern bluebirds

Recent studies suggest that structural plumage coloration can indicate male quality and is used in female mate choice decisions. Whether or not structural coloration functions as a signal in male-male competitive interactions, however, has not been studied. Male eastern bluebirds, Sialia sialis, have brilliant ultraviolet-blue plumage on the head, back, wings and tail that is correlated with both reproductive effort and reproductive success. Bluebirds cannot excavate their own nest cavities, and as a consequence of limited nest sites, often engage in intense competition for nestboxes. We experimentally tested the hypothesis that structural coloration reflects male competitive ability by manipulating the number of available nestboxes. We erected a limited number of nestboxes in early spring and, after birds has established residency in those nestboxes, we added more nestboxes to the study site. We found that the reflective properties of the ultraviolet-blue plumage differed between males that acquired nestboxes early versus late in the spring, indicating that more colourful birds won competitions for access to nest sites. We also found that more colourful males fledged more offspring. These observations support the hypothesis that structural plumage colour is a condition-dependent trait in male eastern bluebirds that could be used to accurately assess the fighting ability of competitors.