Foraging ecology of Herring Gulls (Larus argentatus)
and Great Black-backed Gulls (Larus marinus),
with a focus in New England rocky intertidal

Michelle Rome
Bachelor's of Science in Environmental Science
May 2002

Populations of Great Black-backed Gulls (Larus marinus) and Herring Gulls (L. argentatus) along coastal New England have fluctuated dramatically during the last century. Currently, populations of L. marinus are increasing as L. argentatus are precipitously declining. Studies in North America examining competitive interactions between these two species of gulls have demonstrated that L. marinus displaces L. argentatus from breeding habitats. Quality of nesting and foraging habitat are strong determinates of reproductive success; however, interactions between these two species in foraging habitats have been largely ignored. Within New England, these two species utilize the intertidal as important foraging habitat and can be important intertidal predators within intertidal marine communities. Thus, the foraging behaviors and interactions between L. marinus and L. argentatus within the intertidal have important implications for future gull population trajectories, as well as their potential predatory impacts on marine communities.

In the first chapter, I explore the diet, prey preference, and the intertidal foraging distributions and behaviors for L. argentatus and L. marinus. Diet and prey preferences of the two species greatly overlapped, although L. marinus tended to have a larger proportion of the most preferred prey items. In general, there was an inverse relationship between the density of foraging L. argentatus and L. marinus. All aggressive interspecific interactions were initiated by L. marinus and resulted in movement of L. argentatus from the immediate area. In September, when L. marinus begin to leave the island for overwintering grounds, 1) the size of Cancer borealis increased in L. argentatus remains, 2) the density of L. argentatus foraging and consuming crabs in the intertidal increased, 3) there was a higher proportion of L. argentatus foraging in high density areas and 4) interspecific aggressive interactions decreased.

In the second chapter, I explore the basis of L. argentatus and L. marinus preference for C. borealis. I address three possible hypotheses for the basis of this preference, including: 1) behavioral defenses, 2) crypticity and 3) tissue taste. In tethering experiments, both species of gulls preferred Cancer irroratus without claw defense behavior and Carcinus maenas in patches cleared of algea, indicating that behavioral defenses and crypticity may limit gull capture of C. irroratus and C. maenas. However, when all three species of crabs were tethered 1) without any behavioral defense and 2) in a cleared patch, both species of gulls preferred C. borealis. When the meat of all crab species was offered to L. argentatus and L. marinus in colonies, there was no preference for a crab species. For all tethering experiments, selection choices were not statistically different between L. argentatus and L. marinus. These results indicate that L. argentatus and L. marinus have a very strong preference for C. borealis. Behavioral defenses as well as crypticity may limit capture of C. irroratus and C. maenas, although manipulating these factors does not alter the preference for C. borealis.

In conclusion, these finding suggest that L. argentatus and L. marinus overlap in diet, prey preferences, and the basis of the preference for C. borealis, indicating an overlap in resource utilization and a high potential for competition with intertidal foraging habitats. Furthermore, these findings suggest that aggressive L. marinus, at high densities, may suppress L. argentatus ability to forage and obtain larger C. borealis from the intertidal on Appledore Island.