These birds are large to medium in size with white or gray feathers. Depending on the species, gulls are between 11 and 30 inches in size. Many species have black markings on their heads or wings. Their bills are stout, strong and slightly hooked and their feet are webbed. Their wings are long and pointed, having a wingspan of 3 to 4 feet. They are closely related to terns and distantly to auks. Coloration of many seagulls is seasonal or may change as they mature. During the breeding season, generally adult seagulls have white heads. These include herring gulls, great black-backed gulls, western gulls and glaucous-winged gulls. However, some species, such as Bonaparte's gulls and Franklin's gulls have dark-colored hoods during the breeding season. During winter, gulls with white heads develop streaks on their head, while hooded gull species tend to have a dark spot on the head.