Numbers of Great Black-backed Gulls (Larus marinus) and Herring Gulls (L. argentatus) along coastal New England have changed dramatically during the last century. In recent years, Great Black-backed Gulls have displaced Herring Gulls from optimal breeding sites, thus potentially contributing to the recent decline of the Herring Gull. Although the quality of foraging habitat is also a strong determinate of reproductive success, potential for competition and interactions between these two species in natural foraging habitats have been largely ignored. In this study, the diet and prey preference of the two species were compared. In addition, the spatial and temporal patterns in foraging activities were examined and interspecific interactions in the rocky intertidal zones were also described. Diet and prey preference of the two species greatly overlapped, with both species consuming a large percentage of marine invertebrates (30-60%), especially crabs. Foraging differed, and the Great Black-backed Gull consumed more crabs from the shallow subtidal zone than the Herring Gull. In regions where numbers of gulls were relatively high, Great Black-backed Gulls initiated all aggressive interspecific interactions. In September, when some Great Black-backed Gulls had moved from breeding to overwintering grounds, 1) the size of Jonah Crab (Cancer borealis) carapaces increased in Herring Gull prey, and 2) the numbers of Herring Gulls consuming crabs increased. These findings suggest that during the breeding season, aggression by the Great Black-backed Gull, especially in areas of high numbers, may suppress the ability of the Herring Gull to forage and obtain larger crabs from the shallow subtidal zone at our study site.
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Vol. 27 • No. 2