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, 22–24 cm (8.7–9.4 in) long, with a wingspan of 50 cm (20 in), and weighing 39–52 g (1.4–1.8 oz). 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.
The Least tern, as its name suggests, is the smallest of the Americas’ terns. Groups of them often hover near the surface of the water with their quick, flickering wing beats, catching small fish and invertebrates. They also plunge-dive, as do other terns, hovering above their prey and then suddenly dropping into the sea. Populations of this bird are endangered in many regions due to human impact on nesting areas, particularly competition for the use of beaches. That said, in the east in some areas near the coast these terns are now successfully nesting on gravel roofs.
Least terns breed in North America, as well as locally in the north of South America. They winter mainly along northern South America’s coast, including Brazil to Colombia. They favor open habitat and are typically found near any bodies of water, such as sea coasts, gravel and shell and beaches, islands, bays, lakes, lagoons, estuaries, unobstructed river channels, and salt flats that are associated with reservoirs and rivers.
Least terns fly over water to forage, hovering then plunging to catch prey that are just below the water's surface. Sometimes they will dip down to pick up prey from the surface of the water or from land, and they may catch insects on the wing. During the breeding season, typically they will forage close to the location of their nesting area. They may feed near to the shore of the open ocean - particularly near bay mouths or lagoons. Least terns travel four miles or more (6+ km) from their colony seeking the small fish that from a large part of their diet. This species lives and nests in colonies, parents within a colony defending their nesting territory against intruders. They will fly up into the air when alarmed, making an alarm call, and dive repeatedly at the intruder.
Least terns are carnivores, they mostly eat small fish, like smelt, anchovy, surfperch and silversides (ray-finned fish), and also crabs, shrimp, barnacles, krill and some insects.
These birds are serially monogamous, pairs staying together for the entirety of the breeding season. April to August is the main period for breeding and birds usually remain within the breeding territory between 3 and 5 months. Male least terns engage in a unique courtship ritual which involves a male offering a female food in the hope of her choosing him as a mate. Nests are just shallow scrapes in open sand, pebbles or soil, sometimes lined with grasses or pebbles. Although preferring sandy beaches for their nests, they will occasionally use a flat, gravel-covered rooftop instead. One to three eggs are laid, of a buff or pale green color with dark blotches. Both parents build nests, take part in incubating the eggs over a period of 20-22 days, and look after the young. The downy chicks hatch with their eyes open and can walk soon after. When they are a few days old, they move nearby to short vegetation. They start to fly when they are just under 3 weeks old, and may stay with their parents for as long as 3 months.
Least terns are widespread and are common in places, but the nesting habit they prefer is sought after for residential development, human recreation, and alteration by the diversion of water, which interferes with nesting in many areas. This species for most states in North America is classified as either threatened, endangered, or of concern, due to loss of nesting habitat, the interior population having been federally listed since 1985 as endangered.
According to the New Mexico Avian Conservation Partners resource, the breeding population of the Least tern in North America numbers 60,000-100,000 and the population in New Mexico consists of only several breeding pairs. Overall, currently Least terns are classified as Least Concern (LC), however, their numbers today are decreasing.
