Sex-specific growth in the European Shag Stictocarbo aristotelis, a sexually dimorphic seabird

The European Shag Stictocarbo aristatelis is dimorphic in body size: males are 22% heavier than females. We used molecular techniques to sex of 25 mal e and 18 female chicks in colonies from the Cies Islands (NW Spain). Discri minant functions were then obtained based on morphometric variables (culmen , head, wing and tarsus length) measured throughout the growth of the sexed birds. Once chicks were 25 days of age the discriminant functions accurate ly classified over 95% of cases, and at 30 days of 100% of cases. Using the se functions we retrospectively sexed another 30 males and 35 females to ex amine the growth of the two sexes. The growth of the different variables wa s fitted to a logistic model. Culmen growth was found to be similar in both sexes. The asymptotic head size was larger in males but had the same growt h constant as in females. The wing, tarsus and body mass asymptotes were la rger in males, but females had a higher growth rate. The first principal co mponent (PCI), extracted from a principal component analysis of the morphom etric variables, may be considered as a synoptic descriptor of body size. T he differentiation in mass growth between males and females started when th e birds were 15 days old. However, body size (PC1) was not distinguished un til they were 30 days old. During the first growth stage females were simil ar in body size to males due to their faster growth rate. This would sugges t that the smaller sex (the females) has the same competitive ability as th e larger sex and that the hierarchy in the early stages of growth would be contingent upon hatching order and not sex.