The Black tern is a small graceful member of the family of terns. During the breeding season it is unmistakable, when it displays its distinctive dark plumage after which it is named. Males and females have a similar appearance, with the female being slightly grayer. Except in the breeding season, Black terns are pale gray on top and white on the underside, with a dark patch on their breast. Their head is white with a dark central crown which joins the black patch behind their eye. In front of their eye there is a dark spot as well. During recent decades, Black tern populations have decreased in size in many areas, due to the loss of marsh habitat.
One population is found in Europe, from southern Spain to southern Scandinavia, and eastwards through Asia to central Mongolia. A migratory bird, it winters mainly on the African west coast, as far down as South Africa. There is another population in the Americas, which breeds in Canada and the north of the United States, and winters in Central America and the north of South America. Black terns breed on inland waters like marshes, lakes, pools, or rice fields that have dense marsh and cattail vegetation, or in open water areas. In winter, these birds live mostly offshore, but are also found on the coast, in coastal lagoons and in estuaries. During their migration, they are often seen in estuaries and on coasts, but may also been seen in any wet area, such as ponds and even ditches.
Black terns are diurnal, they are quite gregarious and they usually nest in colonies. During migration they gather in large flocks. Their flight is erratic and buoyant. They are often seen hovering above marshes. They hunt while they are flying, either dipping down to the ground or the water’s surface to pick up food, or chasing insects in the air. Only rarely do they dive into the water to catch prey. Flocks of these birds often gather where dolphins or predatory fish have driven prey up to the water’s surface. They have even been seen chasing minnows that fishermen cast. Black terns defend young in their nest with determination, darting about screaming and making angry swoops at the intruder’s head, often striking it hard with their bills. Their alarm call is the quacking sound of “kek”, made often when intruders approach their nest. Their usual call is a short shrill “kyeh” and a soft, sharp “kik” or “kik-kik”.
Black terns are carnivores and mostly eat insects during the nesting season, and otherwise they also eat small fish, tadpoles, frogs and snails. When migrating, they mainly eat fish, but will also eat insects and crustaceans.
Black terns are monogamous and form long-lasting pair bonds. Breeding takes place in small colonies usually of about 20 pairs, seldom more than 100 pairs. Nests are usually placed more than 3 m from each other. Breeding is between May and June. Black terns usually nest over shallow water, where they build a low mound from plant material on the water or on top of a mat of floating vegetation. An old muskrat house, a shallow scrape on the ground, or the abandoned nest of another water bird can also be used. Nests are often flimsy and are easily destroyed by changing water levels or by wind. The female lays 2-3 eggs and both parents incubate the eggs for around 19 to 23 days. Chicks develop quickly and may leave their nest after only two to three days, remaining in vegetation nearby. They fledge and can fly at about 23 days old, but depend on their parents for food for a further two weeks. These birds leave for their wintering areas from July, and the young birds usually do not return to breed until they are at least two years old.
Black terns are not currently considered under threat of extinction. However, due to a number of threats, many local populations are in decline. These include loss of wetland habitats for drainage and agriculture, human disturbance, degradation of their habitat from the overgrowth of cattails, a reduction in prey due to pollution, lake acidification, pesticides, and exotic fish species being introduced. Overfishing in this species’ winter range may also reduce the availability of prey, and a further threat is outbreaks of avian influenza.
The U.S. Fish & Wildlife Service resource records that at present there are approximately 200 nesting pairs at fewer than 20 of the historic breeding grounds of this species. The All About Birds resource states that the total population of the Black tern in North America is about 100,000-500,000 breeding birds. According to the Royal Society for the Protection of Birds (RSPB) resource, the total Black tern population in Europe is around 83-170,000 pairs. The global population is estimated at 800,000-1,750,000 individuals. Overall, currently Black terns are classified as Least Concern (LC) on the IUCN Red List, however, their numbers today are decreasing.