The increase in gull (Laridae) populations through the 20th century, largely due to an upsurge in anthropogenic food sources, has raised concerns about the effects of gulls on sympatric populations of other seabirds. In Newfoundland and Labrador, Canada, a reduction in fisheries discards due to the collapse of cod (Gadus morhua) populations and a phenological delay in the early 1990s and early 2000s in spawning capelin (Mallotus villosus) has supposedly resulted in increased seabird predation by gulls. Accordingly, the diet of Great Black-backed Gulls (Larus marinus) was quantified at two colonies (Gull Island, Witless Bay, Newfoundland, and Gannet Islands, Labrador), and the total mortality on sympatric breeding seabirds at each site was extrapolated. At the Gannet Islands, Great Black-backed Gulls primarily kleptoparasitized Atlantic Puffins (Fratercula arctica) bringing sandlance (Ammodytes sp.) to their chicks, whereas at Gull Island, seabirds formed the bulk of the gulls' diet. Great Blackbacked Gulls preferred murre (Uria spp.) eggs at both sites, selecting them in far greater proportion than their abundance, consuming up to 40% of Common Murre (U. aalge) eggs laid on Gull Island. Great Black-backed Gulls also targeted Black-legged Kittiwake (Rissa tridactyla) eggs, chicks, and adults at Gull Island. Great Black-backed Gulls clearly selected certain seabird prey disproportionately to their availability, but likely have not had significant effects on the populations of other sympatric seabirds, with the possible exception of Black-legged Kittiwakes, which declined significantly from the 1970s to the early 2000s.