These large terns breed in very dense colonies on coasts and islands, and exceptionally inland on suitable large freshwater lakes close to the coast. They nest in a ground scrape.
Thalasseus terns feed by plunge-diving for fish, almost invariably from the sea. They usually dive directly, and not from the "stepped-hover" favoured by, for example, the Arctic Tern. The offering of fish by the male to the female is part of the courtship display.
These species have long thin sharp bills, usually a shade of yellow or orange except in the Sandwich Tern where the bill is black with a yellow tip in most subspecies. All species have a shaggy crest. In winter, the Thalasseus terns' foreheads become white.
An early Pliocene fossil bone fragment from the northeastern United States closely resembles a modern Royal Tern. It may be an unexpectedly early (3.7–4.8 million years before present) specimen of that species, or an ancestral member of the crested tern group.