The least tern (Sternula antillarum) is a species of tern that breeds in North America and locally in northern South America. It is closely related to, and was formerly often considered conspecific with, the little tern of the Old World. Other close relatives include the yellow-billed tern and Peruvian tern, both from South America.
It is a small tern, long, with a wingspan of , and weighing . The upper parts are a fairly uniform pale gray, and the underparts white. The head is white, with a black cap and line through the eye to the base of the bill, and a small white forehead patch above the bill; in winter, the white forehead is more extensive, with a smaller and less sharply defined black cap. The bill is yellow with a small black tip in summer, all blackish in winter. The legs are yellowish. The wings are mostly pale gray, but with conspicuous black markings on their outermost primaries. It flies over water with fast, jerky wingbeats and a distinctive hunchback appearance, with the bill pointing slightly downward.
It is migratory, wintering in Central America, the Caribbean and northern South America. Many spend their whole first year in their wintering area. It has occurred as a vagrant to Europe, with one record in Great Britain and one in Ireland.
It differs from the little tern mainly in that its rump and tail are gray, not white, and it has a different, more squeaking call; from the yellow-billed tern in being paler gray above and having a black tip to the bill; and from the Peruvian tern in being paler gray above and white (not pale gray) below and having a shorter black tip to the bill.
Subspecies
The differences among the three subspecies may not be as much as had been thought.
S. a. athalassos – (Burleigh & Lowery, 1942): Breeds on the rivers of the Arkansas River, Mississippi River, Brazos River, Trinity River, and Rio Grande basins; winters south to northern Brazil. S. a. antillarum – (Lesson, 1847): nominate, Breeds on the Atlantic coast of North America, from Maine south along the east and south coasts of the United States, Bermuda, the Caribbean, and Venezuela; winters south to northern Brazil. S. a. browni – (Mearns, 1916): California least tern. Breeds on the Pacific coast of North America, from central California south to western Mexico; winters mainly in Central America. Additionally, least terns of an unknown subspecies were found in 2012 nesting on the Big Island of Hawaii.
Conservation and status
S. a. antillarum
The population is about 21,500 pairs; it is not currently considered federally threatened, though it is considered threatened in many of the states in which it breeds. Threats include egg and fledgling predators, high tides and recreational use of nesting beaches.
S. a. athalassos
The interior subspecies, with a current population of about 7000 pairs, was listed as an endangered subspecies in 1985 (estimated 1000 breeding pairs), due to loss of habitat caused by dams, reservoirs, channelization, and other changes to river systems. It was delisted on 13 January 2021.
S. a. browni
The western population, the California least tern, was listed as an endangered species in 1972 with a population of about 600 pairs. With aggressive management, mainly by the exclusion of humans via fencing, the Californian population has rebounded in recent years to about 4500 pairs, a marked increase from 582 pairs in 1974 when census work began, though it is still listed as an endangered subspecies. The California subspecies breeds on beaches and bays of the Pacific Ocean within a very limited range of southern California, in San Francisco Bay and in northwestern Mexico. While numbers have gradually increased with its protected status, it is still vulnerable to predators, natural disasters or further disturbance by humans. Recent threats include the gull-billed tern (Sterna nilotica), which can decrease reproductive success in a colony to less than 10%.