These are omnivores like most Larus gulls, and they will scavenge as well as seeking suitable small prey. They frequently rob other seabirds of their catch. It can swallow a puffin or a small wild duck whole. http://en.wikipedia.org/wiki/Great_Black-backed_Gull
These basically weren't seen in the north New Jersey shore until they opened Arthur Kills landfill.
Flying rats bump