thumb|[[Common tern in flight]]
thumb|Common tern in flight
Terns are seabirds in the subfamily Sterninae of the wider gull and tern family Laridae. They have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated in eleven genera in a subgroup of the family Laridae, which also includes several genera of gulls and the skimmers (Rynchops). They are slender, lightly built birds with long, forked tails, narrow wings, long bills, and relatively short legs. Most species are pale grey above and white below with a contrasting black cap to the head, but the marsh terns, the black-bellied tern, the Inca tern, and some noddies have dark body plumage for at least part of the year. The sexes are identical in appearance, but young birds are readily distinguishable from adults. Terns have a non-breeding plumage, which usually involves a white forehead and much-reduced black cap.
Terns are long-lived birds and are relatively free from natural predators and parasites; most species are declining in numbers due directly or indirectly to human activities, including habitat loss, pollution, disturbance, and predation by introduced mammals. The Chinese crested tern is critically endangered and three other species are classed as endangered. International agreements provide a measure of protection, but adults and eggs of some species are still used for food in the tropics.
Description
thumb|The plumage of the [[Inca tern is the most atypical of the group.]]
Terns range in size from the least tern, at in length and weighing , to the Caspian tern at , . They are longer-billed, lighter-bodied, and more streamlined than gulls, and their long tails and long narrow wings give them an elegance in flight. Male and female plumages are identical, although the male can be 2–5% larger than the female and often has a relatively larger bill. Sea terns have deeply forked tails, and at least a shallow "V" is shown by all other species. Although their legs are short, terns can run well. They rarely swim, despite having webbed feet, usually landing on water only to bathe. Plumage type, especially the head pattern, is linked to the phylogeny of the terns, and the pale-capped, dark-bodied noddies are believed to have diverged earlier than the other genera from an ancestral white-headed gull, followed by the partially black-headed Onychoprion and Sternula groupings.
Juvenile terns typically have brown- or yellow-tinged upperparts, and the feathers have dark edges that give the plumage a scaly appearance. They have dark bands on the wings and short tails. In most species, the subsequent moult does not start until after migration, the plumage then becoming more like the adult, but with some retained juvenile feathers and a white forehead with only a partial dark cap. By the second summer, the appearance is very like the adult, and full mature plumage is usually attained by the third year. After breeding, terns moult into a winter plumage, typically showing a white forehead. Heavily worn or aberrant plumages such as melanism and albinism are much rarer in terns than in gulls.
Voice
Terns have a wide repertoire of vocalisations. For example, the common tern has a distinctive alarm, kee-yah, also used as a warning to intruders, and a shorter kyar, given as an individual takes flight in response to a more serious threat; this quietens the usually noisy colony while its residents assess the danger. Other calls include a down-slurred keeur given when an adult is approaching the nest with a fish, and a kip uttered during social contact. Parents and chicks can locate one another by call, and siblings also recognise each other's vocalisations from about the twelfth day after hatching, which helps to keep the brood together.
Vocal differences reinforce species separation between closely related birds such as the least and little terns, and can help humans distinguish similar species, such as common and arctic terns, since flight calls are unique to each species. Early authors such as Conrad Gessner, Francis Willughby, and William Turner did not clearly separate terns from gulls, Behaviour and morphology suggest that the terns are more closely related to the gulls than to the skimmers or skuas, and although Charles Lucien Bonaparte created the family Sternidae for the terns in 1838, for many years they were considered to be a subfamily, Sterninae, of the gull family, Laridae. Relationships between various tern species, and between the terns and the other Charadriiformes, were formerly difficult to resolve because of a poor fossil record and the misidentification of some finds.
Following genetic research in the early twenty-first century, the terns were historically treated as a separate family, Sternidae. Most terns were formerly treated as belonging to one large genus, Sterna, with just a few dark species placed in other genera; in one 1959 paper, only the noddies and the Inca tern were excluded from Sterna. A recent analysis of DNA sequences supported the splitting of Sterna into several smaller genera. One study of part of the cytochrome b gene sequence found a close relationship between terns and a group of waders in the suborder Thinocori. These results are in disagreement with other molecular and morphological studies, and have been interpreted as showing either a large degree of molecular convergent evolution between the terns and these waders, or the retention of an ancient genotype. a taxonomy that was followed by the IOC World Bird List for several years up to 2023, but more comprehensive analysis has now shown that the noddies are basal to only the other terns, not the whole family; As now, the term was used for the inland black tern as well as the marine species. Some authorities consider "tearn" and similar forms to be variants of "stearn", Linnaeus adopted "stearn" or "sterna" (which the naturalist William Turner had used in 1544 as a Latinisation of an English word, presumably "stern", for the black tern) or a North Germanic equivalent for his genus name Sterna. All of these names are ultimately onomatopoeic, derived from the bird's calls.