Evolution of reversed sexual size dimorphism in skuas and jaegers

Skuas and jaegers (Stercorariinae), like other birds with a raptorial lifes tyle, display reversed sexual dimorphism in size (RSD) in which females are larger than males. This condition is commonly believed to be associated wi th the morphology or behavioral ecology of avian predators, and numerous th eoretical explanations have been presented to account for its evolution. Mo st tests of these ideas have been theoretical or based on comparative analy ses. We present tests of specific hypotheses of RSD based on field studies of Great Skuas (Catharacta skua) and Parasitic Jaegers (Stercorarius parasi ticus) breeding in Shetland. We also present a comparative analysis of fora ging behavior and degree of RSD within the Stercorariinae. We found no supp ort for the popular theories that claim that RSD evolved as a result of spe cialized roles during breeding. Large females were not more fecund, nor wer e they the main defenders of the nest. Male body size was not correlated wi th any breeding parameter. In addition, interspecific comparisons did not s upport the idea that the degree of RSD is related to foraging behavior duri ng the breeding season (as opposed to morphology). Female dominance within pairs did not affect any breeding parameters. Support was found for theorie s that RSD is linked to sexual selection. Large females had an advantage wh en pairing with older males, which could be a result of intrasexual competi tion for mates. There was assortative mating for size, indicating that smal l males could also have an advantage in pairing, because large males seem t o be rejected by small females. Given the disparate results of several prev ious studies, we conclude that a large body of empirical evidence will be n eeded before we can draw firm conclusions about the main selective factors that act to maintain current levels of RSD in natural populations of raptor ial birds.