Foraging Ecology of Great Black-backed and Herring Gulls on Kent Island in the Bay of Fundy

Abstract

I studied the foraging ecology of the generalist predators Great Black-backed (Larusmarinus) and Herring (Larusargentatus) Gulls on Kent Island, in the Bay of Fundy. To study diet, I collected pellets casted in and around nests supplemented with tissue samples (red blood cells, plasma, head feathers and primary feathers) obtained from chicks and adults for stable isotope analysis. I collected 51 pellets from Herring Gulls and 31 from Great Blackbacked Gulls. 12 adult and 12 chick Great Black-backed and 38 adult and 12 chick Herring Gulls were captured and sampled. Pellets indicated that Herring Gulls predominantly feed on fish and crab and Great Black-backed Gulls on crab and fish. There were consistent differences in isotopic signatures between species, age and tissue types. All of the adult Great Black-backed Gull δ15N levels were higher than for adult Herring Gulls. Adults of both species also had higher δ15N levels than the chicks and Great Black-backed Gull chicks were higher than Herring Gull chicks. Thus Great Black-backed Gulls feed at a higher trophic level than Herring Gulls and adults at a higher trophic level than the chicks. Adult Great Black-backed Gulls had higher δ13C levels than Herring Gulls for compact red blood cells and primary feathers. Herring Gull δ13C levels decreased over the May and June, but no significant change was seen in δ15N levels. Adults of both species were estimated to feed their chicks more krill and mackerel than they feed on themselves. Given their broad diet and reliance of fisheries offal, gulls were determined to be more suited towards monitoring for pollutants in the marine environment.