In the past often combined with the family Laridae, which includes the 51 species of gulls, the Sternidae is now usually treated as a separate family due to the many morphological and ecological differences between these two groups. The terns are a truly cosmopolitan, breeding on all continents, including Antarctica. Whereas the gulls are common in the temperate regions, especially that of the Northern Hemisphere, and rare in the tropics, most of the terns either migrate to the tropics or Southern Hemisphere and many breed in the tropics. The vast majority of the 44 tern species breed on rocky and sandy coasts on the mainland and inshore archipelagos, while others breed on only on oceanic islands, and several breed on inland rivers or marshes. Outside their breeding seasons most inland breeders move to the coast, while some coastal breeders become entirely pelagic, spending months at sea.
Terns are much more slender and streamlined than gulls, with characteristically long, narrow, pointed wings, and long, forked tails, often with elongated tail streamers. They feed mainly on schooling fish, which are often caught by plunge-diving, in which the bird first hovers over the prey, then dives headlong into the water, usually submerging completely, before rising up into the air with its catch. When fish or other prey, such as crustacenas and aquatic insects, are close to the water’s surface, they may forage by repeatedly swooping down and dipping the bill beneath the surface. Insects emerging from freshwater marshes are pursued on the wing by some species. Terns are generally gregarious, breeding in dense colonies, and often congregating at sea to feed on fish shoals. When not foraging, they usually rest on reefs, sand-bars and beaches, frequently in the company of other tern and shorebird species.
No fewer than 15 species of terns have been recorded in Java, and of these are known to visit Baluran
Typically buoyant and graceful, with fairly constant wing beats; skillful and agile fliers, able to hover or skim over water when fishing, or circle and wheel in pursuit of aerial insects.