• A large tern with a white forehead black cap that crests behind the head
Caspian Tern is much larger with bright red bill. Lesser Crested Tern is smaller, paler above and has an orange, not yellow bill. Royal Tern is much paler above and has a rich orange bill. In breeding plumage, Great Crested Tern has a white forehead, while Royal Tern has the forehead black connecting to the black cap.
From opus at www.birdforum.net