Relationships between bird morphology and prey selection in two sympatric Great Cormorant Phalacrocorax carbo subspecies during winter

Variation in bird morphology (notably sex size dimorphism) has been suggested to contribute to differences in food use between individuals. We explore the hypothesis of food partitioning (diet overlap and prey size selection) in two sympatric subspecies of the Great Cormorant Phalacrocorax carbo with respect to bird morphology (subspecies and sex) in inland French waters. These areas represent a recent and increasing contact zone used as a common non-territorial winter feeding area by the continental P. c. sinensis and the marine P. c. carbo subspecies. A high dietary overlap between subspecies and sexes was found at the major sites studied. Prey size selection was found to be site-specific and generally related to a gradient of structural size and body mass (male P. c. carbo > male P. c. sinensis > female P. c. carbo > female P. c. sinensis). With respect to bird morphology, differences in prey size consumed by Great Cormorants were more pronounced between sexes in freshwater habitats than between subspecies. This was reinforced by the fact that P. c. carbo birds entering inland areas were smaller than birds on the coast. These results also suggest that bird morphology is an important determinant of dietary differences. Our study demonstrated that P. c. carbo is an efficient generalist forager in continental areas, and food partitioning in type and size of prey between the two subspecies is low.