The common tern (Sterna hirundo ) is a seabird in the family Laridae. This bird has a circumpolar distribution, its four subspecies breeding in temperate and subarctic regions of Europe, Asia and North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. Breeding adults have light grey upperparts, white to very light grey underparts, a black cap, orange-red legs, and a narrow pointed bill. Depending on the subspecies, the bill may be mostly red with a black tip or all black. There are a number of similar species, including the partly sympatric Arctic tern, which can be separated on plumage details, leg and bill colour, or vocalisations.
Breeding in a wider range of habitats than any of its relatives, the common tern nests on any flat, poorly vegetated surface close to water, including beaches and islands, and it readily adapts to artificial substrates such as floating rafts. The nest may be a bare scrape in sand or gravel, but it is often lined or edged with whatever debris is available. Up to three eggs may be laid, their dull colours and blotchy patterns providing camouflage on the open beach. Incubation is by both sexes, and the eggs hatch in around 21–22 days, longer if the colony is disturbed by predators. The downy chicks fledge in 22–28 days. Like most terns, this species feeds by plunge-diving for fish, either in the sea or in freshwater, but molluscs, crustaceans and other invertebrate prey may form a significant part of the diet in some areas.
Eggs and young are vulnerable to predation by mammals such as rats and American mink, and large birds including gulls, owls and herons. Common terns may be infected by lice, parasitic worms, and mites, although blood parasites appear to be rare. Its large population and huge breeding range mean that this species is classed as being of least concern, although numbers in North America have declined sharply in recent decades. Despite international legislation protecting the common tern, in some areas populations are threatened by habitat loss, pollution or the disturbance of breeding colonies.
Known for its attractive plumage and graceful flight, Common terns have a slender body and a smoothly rounded head, and long pointed wings. Its breeding plumage is light silvery-gray upperparts and clear black outer primaries on its wingtips. Outside the breeding season, the bird keeps some of its distinguishable black cap, but its forehead and face turn white. Its legs and bill turn black, losing their orange-red coloration. Male and female are similar in appearance throughout the year.
Common terns occur from northern Canada to the Caribbean Sea in the south, throughout Europe, Northern Asia, the Middle East and North Africa. Outside of the mating season, these birds winter along the coasts of South America and Central America, along Africa’s coast and the Arabian Peninsula, as well as on islands in the Indian Ocean and throughout Australasia and much of Southeast Asia. They breed on sand spits, beaches and low-lying inshore islands. They also nest inland near slow-flowing rivers, and lakes in open country. This species favors areas that have close shallow waters where they can fish. During migration, they are mainly seen along coasts, at freshwater inland lakes and in estuaries. During winter, they are mostly coastal, in warm tropical and subtropical waters.
Common terns are diurnal birds and they live in colonies with no clear hierarchy among them, with all seeming to be equal. Although Common terns all migrate and live together, the family unit is responsible to feed and care for its own eggs and chicks, and individuals will often defend feeding territories. This species forages by flying above water and hovering, often plunge-diving to catch prey under the surface. It will also swim as it picks up food from the surface of the water or just below it. A highly migratory bird, it leaves the breeding grounds for the wintering sites typically from August to October, soon after all the chicks fledge. These birds communicate mostly by means of their unusual, hoarse voices, having three different and distinct calls. Communication during mating is mainly visual and/or tactile.
Common terns are carnivores (piscivores), they mainly eat fish, but also consume shrimps and other crustaceans, small squid, marine worms, and leeches.
Common terns are monogamous, which means they mate with only one partner during a breeding season. At the time of courtship, which starts in April, males establish their territories within the colony before commencing what is known as "courtship feeding," where a male brings a fish to a female to court her. Courtship displays include the male posturing followed by the pair circling each other. Eggs are laid from April to June. Common terns usually breed in huge colonies, though there may be some isolated pairs. Both parents construct their nest on bare ground, sometimes with low vegetation surrounding it or on dead floating vegetation. 2-3 eggs are laid and are incubated by both parents for 21-25 days. Chicks leave their nest after several days, but remain nearby and their parents still feed them. They fledge at around 22-28 days old, remaining in the family group for at least another two months. Common terns become reproductively mature at 3 years of age.
The Common tern enjoys a wide range, but its colonies are threatened by human disturbance through increase in recreational activities, naturally fluctuating water-levels flooding their nest sites, habitat loss from coastal development, erosion, pollution and invasive vegetation at nesting sites.
According to IUCN’s Red List, the total population size of the Common tern is 1,600,000-3,600,000 individuals. National population sizes were estimated at 10,000-100,000 breeding pairs; in China, 1,000-10,000 individuals migrating and 50-1,000 wintering, and in Russia, 10,000-100,000 breeding pairs and 1,000-10,000 migrating individuals. Overall, currently Common terns are classified as Least Concern (LC), however their numbers today are decreasing.
Common terns impact the populations of prey that they eat and act as an important source of food for their predators. Sometimes the uneaten fish they catch are eaten by scavenging animals that live in the same area.
