svartbak Larus marinus

The largest gull in the world. Huge, bulky, and large-billed. Adults have dark black backs and pale pink legs. Nonbreeding adults have very limited streaking on the head. Takes four years for immatures to reach adult plumage; intermediate plumages are often mottled and messy. Tends to look colder-toned overall, with more checkered upperparts and a whiter head than other immature gulls. Always note large size and thick bill. Most common along the coasts of the northeastern U.S. and Europe, but also occurs on large lakes (especially the Great Lakes). Often gathers in mixed flocks with other large gulls on beaches and mudflats.