Diet of Imperial Cormorants (Phalacrocorax atriceps) and Rock Shags (P. Magellanicus) breeding sympatrically in Patagonia, Argentina

We analyzed the diet of Imperial Cormorants (Phalacrocorax atriceps) and Rock Shags (P. magellanicus) breeding sympatrically at Malaspina Inlet, Patagonia, Argentina, in order to assess food partitioning between them. Sixty-four regurgitations were collected during the 1999 breeding season on two colonies separated by 2.2 km. Both species shared many of the identified prey items, mainly fish arid crustacean. However, Imperial Cormorants fed on a wider variety of preys than Rock Shags did. Moreover, the latter preyed mainly on benthic fish whereas Imperial Cormorants also consumed pelagic fishes, thus foraging in most of the water column. Among fish, Notothenia sp., followed by Helcogrammoides cunninghami, predominated in the diet of both cormorants, whereas Imperial Cormorants also preyed on Engraulis anchoita and Merluccius hubssi. The spatial distribution of diet samples showed a clear overlap in the food resources used by both cormorants as well as a convergence in the fish composition of the diet. The observed overlap in the diet was in concordance with the relative overlap in the foraging areas used by both species, as reported in previous studies, thus probably reflecting good food availability in waters surrounding, Malaspina Inlet. Therefore, the mechanisms of a possible ecological segregation that resulted in the documented differences in the diet of both species could have arisen from a combination of dissimilarities in diving strategies and foraging behavior.