Morphometric similarities between central and peripheral populations of the European shag Phalacrocorax aristotelis

Martínez-Abraín, A., Oro, D., Velando, A., Genovart, M., Gerique, C., Bartolomé, M.A., Sarzo, B. & Villuendas, E., 2006. Morphometric similarities between central and peripheral populations of the European shag Phalacrocorax aristotelis. Marine Ornithology 34: 21–24.

We compared morphometrics and discriminant functions for sexing European Shags Phalacrocorax aristotelis in a central (British) population and a peripheral (northwestern Iberia) population within its distribution range, to account for structural differences between populations and sexes. Overall, British shags were structurally larger than Iberian shags, except in their head and bill length. This north–south cline could be attributable to higher food availability or lower ambient temperatures at higher latitudes. Furthermore, Iberian male shags were structurally larger than females for all variables considered, except for bill length, which was similar for both sexes, as in the British population. This suggests that bill length is a conservative trait in the species, being similar between populations and sexes alike. The most parsimonious discriminant function for sexing Iberian shags included only bill depth, and it correctly sexed 92.6% of the original cases, as was the case among British shags. This suggests that the depth of the bill likely plays a relevant sex-specific role in the species. Applying discriminant functions derived from one population to other populations can be problematic. However, we found that the bill depth discriminant function for British shags correctly sexed 90.2% of Iberian shags, supporting the idea that, although British shags are larger than northwestern Iberian shags, differences between sexes are of the same magnitude.

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

Velando, A., Graves, J. & Freire, J. Sex-specific growth in the European shag Stictocarbo aristotelis, a sexually dimorphic seabird. Ardea 88, 2 (2000).

The European Shag Stictocarbo aristotelis is dimorphic in body size: males are 22% heavier than females . We used molecular techniques to sex of 25 male and 18 female chicks in colonies from the Cíes Islands (NW Spain). Discriminant 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 accurately classified over 95% of cases, and at 30 days of 100% of cases. Using these functions we retrospectively sexed another 30 males and 35 females to examine the growth of the two sexes. The growth of the different variables was 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 growth constant as in females. The wing, tarsus and body mass asymptotes were larger in males, but females had a higher growth rate. The first principal component (PC I), extracted from a principal component analysis of the morphometric variables, may be considered as a synoptic descriptor of body size. The differentiation in mass growth between males and females started when the birds were 15 days old. However, body size (PC1) was not distinguished until they were 30 days old. During the first growth stage females were similar in body size to males due to their faster growth rate. This would suggest that the smaller sex (the females) has the same competitive ability as the larger sex and that the hierarchy in the early stages of growth would be contingent upon hatching order and not sex .