Ornamental plumage coloration and condition are dependent on age in eastern bluebirds Sialia sialis

Abstract

Male eastern bluebirds Sialia sialis have striking ultraviolet (UV)‐blue coloration on their heads, backs, rumps, wings, and tails and bold chestnut coloration on their breasts. These colored areas are ornaments that correlate with pairing date and reproductive effort, and thus probably influence the choice of mates by females. Such ornaments are expected to increase in color with age and body condition. We investigated the effects of age on body condition and the UV‐blue and chestnut coloration of males over four years using both cross‐sectional (comparing age classes) and longitudinal analyses (following individuals as they age). We found that both the body condition index and brightness of UV‐blue rump coloration increased with age, while UV‐blue tail plumage coloration increased between yearling and older males, and the chestnut breast coloration decreased in the oldest age class. The proximate mechanisms whereby individuals reliably signal age via rump brightness and tail coloration are probably different. Contour feathers, including rump feathers, are molted at approximately the same time in all age classes and are likely subject to the same production costs in all age classes. In contrast, the molt schedule of the tail and wing feathers differs between individuals of yearling and older age classes, with yearlings retaining wing and tail feathers for several months longer than adults. The relationship between tail color and age was probably, in part, a consequence of yearlings expressing tails that have increased feather wear and accumulation of dirt. In general, UV‐blue coloration increased with age while chestnut plumage decreased with age, indicating that older individuals may tradeoff investing energy in structural and melanin ornaments. By assessing overall plumage coloration, female eastern bluebirds could estimate age class when choosing mates.