# Thalasseus

Thalasseus, the crested terns, is a genus of eight species of medium-large to large terns in the family Laridae.
The species have a worldwide distribution in temperate and tropical seas, mostly between about 43° N and S latitude, but to 60° N in the warm waters of the North Atlantic Current in the eastern North Atlantic Ocean; they do not occur in colder arctic or antarctic waters. Several of the species are abundant and well-known birds in their ranges; one is however extremely rare and critically endangered. This genus had originally been distinguished by Friedrich Boie in 1822, but had been little used (with one exception in 1978) until a 2005 study confirmed the need for a separate genus for the crested terns.
Thalasseus terns are large for terns, from 35–53 cm long, with lesser crested tern marginally the smallest, and greater crested tern marginally the largest. The underside plumage is white in all species, while the wings and back vary from pale silvery grey to dark grey. They have long thin sharp bills, a shade of yellow or orange except in the Sandwich tern and Cabot's tern where the bills are black with yellow tips (variably more extensively yellow in one subspecies of Cabot's tern). All species have a shaggy black crest, which is erectile and used in the courtship display. In winter, the foreheads become white to a variable extent. They 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.
Their habit of breeding in very dense colonies made them highly vulnerable to the 2021–2023 highly pathogenic avian influenza (HPAI) outbreaks, with mass mortality in numerous colonies of Sandwich tern in particular.

## Taxonomy

The genus Thalasseus was described by the German zoologist Friedrich Boie in 1822. The type species was subsequently designated as the sandwich tern (Thalasseus sandvicensis). The generic name is derived from the Ancient Greek Thalassa meaning "sea".

### List of species

The genus contains eight species:
| Name | Common name | Distribution |
| Thalasseus sandvicensis | Sandwich tern | Northern Europe to Mediterranean, Black and Caspian Seas, wintering south to South Africa and Sri Lanka. Monotypic. |
| Thalasseus acuflavidus | Cabot's tern | East coast of the Americas from New Jersey south to Chubut, Argentina, also wintering on the Pacific coast. Two subspecies, T. a. acuflavidus (North America) and T. a. eurygnathus (South America). |
| Thalasseus elegans | Elegant tern | Southern California, USA and western Mexico and wintering south to Peru, Ecuador and Chile. Monotypic. |
| Thalasseus bengalensis | Lesser crested tern | Southern Mediterranean and Red Seas across the Indian Ocean to the western Pacific, and Australia, also wintering on African west coast south to Senegal. Three subspecies, T. b. bengalensis (Indian Ocean), T. b. emigratus (Mediterranean) and T. b. torresii (seas around Australia). |
| Thalasseus albididorsalis | West African crested tern | Coasts of Mauritania to Guinea, wintering north to Morocco and south to Angola. Monotypic. |
| Thalasseus maximus | Royal tern | Coasts of the Americas, from Virginia, USA south to Chubut, Argentina in east, and California south to Peru in west. Monotypic. |
| Thalasseus bergii | Greater crested tern | From South Africa around the Indian Ocean to the central Pacific and Australia. Four subspecies, T. b. bergii (southern Africa), T. b. thalassinus (eastern Africa), T. b. velox (northern Indian Ocean), and T. b. cristatus (western Pacific Ocean). |
| Thalasseus bernsteini | Chinese crested tern | Fujian Province, China, and wintering south to the Philippines. Monotypic. Critically endangered. |
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.