Most terns inhabit shores and marshes, but the Sooty tern lives as a true seabird. It is a long-winged flier, and wanders tropical oceans, coming to nest on remote islands. Noisy in its nesting colonies during both the day and at night, one name sailors call it is "wide-awake." Young terns leave the breeding grounds to return after several years. They do not land on the water to rest, and only rarely on floating objects, living in the air year after year. They feed while flying, scooping up their prey from the surface of the ocean, sometimes capturing fish that other predators have chased out of the water.
The Sooty tern inhabits the subtropical and tropical waters of the Americas, Asia, Africa, and Australia. They leave their breeding colonies when the chicks fledge. Terns from the West Indies disperse into the Gulf of Mexico and the Caribbean, where they may spend the winter. Juveniles move south across the Atlantic Ocean towards the African coasts. The Indian Ocean populations probably disperse widely within their range. These birds occur in waters that are rich in fish, squid and plankton, avoiding the cold-current areas. Their breeding grounds are on islands out in the ocean, or barrier islands of rock, sand, or coral, which are usually open and flat, and often heavily vegetated. Artificial island structures are sometimes used. Outside the breeding season, these birds are highly pelagic. After storms and hurricanes they can be vagrant inland.
Sooty terns feed by means of several techniques, including aerial-dipping, skimming over the water or contact-dipping. They rarely use plunge-diving. They catch flying fish in the air. They also feed at night, catching fish that come to the surface when it is dark. Breeding terns eat enough for two, bringing back food for the young chick. These birds cannot swim and do not have water-repellent plumage, and so rarely rests on water, or they would become waterlogged and not able to lift off. It will take naps lasting 1-2 seconds while it is flying, allowing it to stay up in the air. From the end of fledging until first breeding (4-5 years), an individual does not visit land and it goes to islands only to nest. These terns nest close to each other, even when there is more space available. With neighboring birds so near by, pecking and bickering often happens between adults, and even between adults and the neighboring young from another pair. They also fiercely defend their colony by attacking intruders from the air.
Sooty terns are monogamous breeders. This means that males will mate with only one female and females will mate with only one male. The breeding season is dependent on the location of the particular nesting site, some being used for year-round breeding. The Sooty tern can breed every 6-7 months in some locations, whereas Caribbean colonies have a cycle of a year. During the courtship displays, mates will circle around each other, their wings slightly dropped and their bill pointing downwards, calling softly. These birds breed on islands in huge colonies, with some breeding sites occupied over several centuries. Nests are on flat sites like open sand, rock, shell, or coral with vegetation. Laying, however, is on bare ground. Nests are often just a slight depression, with nests about 50 cm apart. Usually a single egg is laid, whitish in color. Incubation is for 28-30 days, with both parents taking turns. Chicks are fed regurgitated food and they fledge in two months. Sooty terns reach maturity at about 6 years of age.
Populations of Sooty terns are not currently threatened, but predation by introduced rats and cats in some areas is a threat. These birds are vulnerable to climatic change, weather conditions, oil pollution, and persistent egg collection in some areas, causing them to move to less suitable sites where there is reduced breeding success.
This bird is exceptionally common. The IUCN Red List reports that the world’s Sooty tern population is estimated to be about 21,000,000-22,000,000 individuals. In Japan the population has been estimated to be no more than 100,000 breeding pairs, with fewer than 1,000 individuals in migration. Currently sooty terns are classified as Least Concern (LC).