Gull diets reveal dietary partitioning, influences of isotopic signatures on body condition, and ecosystem changes at a remote colony

Abstract

As top predators that feed on a wide range of prey items, gull diets may serve as important biological indicators of regional prey availability and changes in marine ecosystems. We studied the diets of herring gulls Larus argentatus and great black-backed gulls L. marinus on Sable Island, Nova Scotia, Canada, a remote colony which has shown high levels of contaminants in herring gull eggs and which has experienced significant ecological and anthropogenic change in its surrounding marine region over the past 40 yr. Analysis of regurgitated pellets suggested that current gull diets have proportionally less offshore prey (e.g. fish) and terns and tern eggs, and proportionally more molluscs, rock crabs Cancer borealis, and seal Halichoerus grypus car- rion than diets sampled 40 yr ago. The composition of recent diets observed from pellet analysis is supported by stable isotope mixing models of carbon (δ 13 C) and nitrogen (δ 15 N), which revealed that great black-backed gulls had high proportions of seals and crab in their diets, whereas her- ring gulls had high proportions of crab, sand lance Ammodytes sp., and terrestrial invertebrates. Isotopic analyses also identified dietary variability through seasonal, age-specific and body condi- tion relationships for each species. Biometricisotope relationships showed that larger great black-backed gulls fed at higher trophic levels, and that higher trophic level foraging in herring gulls was associated with better body condition. Collectively, these results indicate dietary parti- tioning within this community of sympatrically nesting gulls, and broad-scale dietary shifts since the early 1970s.