Nape Coloration Varies with Sex, not Age, Among Great Shearwaters <i>Ardenna gravis</i>

Most petrels (family Procellariidae) exhibit little or no obvious variation in plumage with age or sex, either because plumage performs no sexual function or does so in a way poorly perceptible to humans. This limits the inferences that can be made from visual observations of petrels at sea. However, it has been suggested that nape coloration of Great Shearwaters Ardenna gravis whitens with age. Here we test this supposition using observations of known-age-class individuals. We necropsied birds bycaught around Gough Island, a major breeding colony in the South Atlantic Ocean, and in Massachusetts Bay, a wintering area off the northeastern coast of the USA, to determine sex and classify nape coloration. In addition, we classified the nape coloration of adults and fledglings photographed in colonies on Gough Island and Inaccessible Island in the South Atlantic. Across birds (n = 328), ratios of light:intermediate:dark napes did not differ significantly between age classes, and the accuracy of age classification based on putative nape variation was only 52%. Nape coloration did, however, vary systematically with sex and location: in Massachusetts Bay, light napes were more prevalent in adult females. Off Gough Island, where only adults were sampled, this disparity did not occur. We conclude that while nape coloration may vary due to feather wear, it is not a reliable indicator of age. Rather, it may perform a sexual function, possibly mediating mate choice. Further study of plumage variation and behavior at the colony would be required to test this hypothesis.