Sex and age dependency of wing patch size in Brown Skuas

Abstract

The white wing patches of the uniform brown skuas of the genus Catharacta are the most conspicuous plumage characteristics. They are assumed to be signals between individuals during aggressive and mating behaviour. We investigated the relationship between wing patch characteristics and individual traits of 118 Brown Skuas (Catharacta antarctica lonnbergi) from an Antarctic breeding site. The wing patches are formed by the basal parts of generally nine primaries. Patch size was sex dependent, with patches being, on average, 18% larger in females (85.4 cm²) than in males (72.4 cm²). Age and patch size were positively correlated and the size increased until an age of 13 years. Neither body condition of non-breeders and breeders nor the reproductive success of breeding birds was related to patch size, but breeding birds of both sexes had larger wing patches than non-breeding birds. The results support the hypothesis that wing patches in Brown Skuas act as signals for reproductive ability but not as ornaments coding individual quality for body condition and reproductive performance.