Wikipedia
Thalasseus, the crested terns, is a genus of six species of seabirds in the tern family. Thalasseus signifies a "creature of the sea". It has a worldwide distribution, and many of its species are abundant and well-known birds in their ranges. This genus had originally been created by Friedrich Boie in 1822, but had been abandoned until a 2005 study confirmed the need for a separate genus for the crested terns.
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.