Tern, a large migratory waterbird related to the gulls. Most terns are seabirds, but some also frequent freshwater and a few are restricted to freshwater. Although most terns are found close to shore, some wander far out to sea. Terns are found in virtually all regions of the world. Because of their graceful flight, they are often called sea swallows.

There are 39 species of terns, of which about a dozen are fly found in the United States. Terns range in length from 8 to about 23 inches (20 to 58 cm). All terns have straight, pointed bills, and most species have forked tails. Most terns are white with gray back and black head cap. However, the black tern is almost entirely black, and the fairy tern is completely white. Both sexes have the same coloring. Most terns eat fish or other water animals that they catch by diving into the water. Some species eat insects. All species have similar sharp, rasping cries.

Terns breed when they are two to three years old. At mating time, each species gathers in colonies, and almost all species nest on the ground—sometimes on bare rocks or sand and sometimes filling a hollow with seaweed, sticks, grass, or moss. The female lays up to four whitish, brownish, or greenish speckled eggs. Both sexes incubate the eggs and feed the young. The incubation period ranges from 21 to 28 days.

The common tern is found in north temperate regions around the world. It migrates south below the Equator in both the Eastern and the Western Hemisphere. The common tern is about 15 inches (38 cm) long and has a white body with gray back and wings and a deeply forked white tail. The top of its head is black and its bill is red with a black tip. Its feet are red.

The common ternThe common tern has a gray-white body and a deeply forked tail.

The Arctic tern is famous for its long migratory flight. After breeding near the North Pole, it migrates to antarctic regions, where it spends the winter—a round trip of about 22,000 miles (35,000 km). The Arctic tern is similar in coloring to the common tern, but generally lacks the black tip on the bill.

The largest tern is the Caspian tern, which can reach a length of almost two feet (60 cm). This tern is found in almost all areas of the world. The smallest species is the least tern. It is about eight to nine inches (20 to 23 cm) in length and has a distinctive black-tipped, yellow bill. It is found in most parts of the world.

Of the 39 species of terns, most belong to the genus Sterna. The black tern is Chlidonias niger; the fairy tern, Gygis alba; the common tern, S. hirundo; the Arctic tern, S. paradisaea; the Caspian tern, Hydroprogne caspia; the least tern, S. albifrons. All belong to the family Laridae.