THE FOODS OF GREAT AND SOOTY SHEARWATERS PUFFINUS GRAVIS AND P. GRISEUS IN EASTERN CANADIAN WATERS

Summary The foods of Great and Sooty Shearwaters Puffinus gravis and P. griseus are described from birds collected off eastern Canada. There was a broad overlap in diet, but Great Shearwaters tended to take more squid and tough-bodied fish such as mackerel Scomber scombrus while Sooties took more euphausiids Meganyctiphanes norvegica and soft-bodied fish such as Herring Clupea harengus. These differences are apparently related to differences in bill structure and in the degree of adaptation to underwater swimming. The birds appeared to feed opportunistically on whatever prey was locally available in the size-range between euphausiids and small fish and squid: Meganyctiphanes off southwest Nova Scotia, spawning and post-spawning capelin Mallotus villosus off eastern Newfoundland, and migrating squid Illex illecebrosus on the Grand Bank. Possible factors influencing prey selection are discussed. It is suggested that the timing and routes of the birds' migrations in the North Atlantic are related to the exploitation of such local concentrations. Despite the overlap in diets, differences in the distributions of the two species rule out the possibility of significant competition for common food resources.