Gulls eat fish, mollusks, crustaceans, insects, worms, eggs and rodents and they scavenge food from humans. In fact, gulls are experts at it and easily become accustomed to getting easy meals. Easy meals are food items obtained from trash cans and other discarded human food. It is not uncommon to see gulls at the beach or in urban areas stealing food from trash cans or picnic sites. Sometimes gulls steal food from each other or other species of birds.
Courtesy of ehow.com