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

We compared morphometrics and discriminant functions for sexing European Shags Phalacrocorax aristotelis in a central (British) population and a peripheral (NW 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 due 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 both between populations and sexes. The most parsimonious discriminant function for sexing Iberian shags included only bill depth and 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 NW Iberian shags, differences between sexes are of the same magnitude.

MARTINEZ-ABRAIN, A., ORO, D. et al., Marine Ornithology 34: 21-24