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
Terns range in size from the least tern, at in length and weighing ,Maehr & Kale (2005) p. 111.Olsen & Larsson (1995) p. 136. to the Caspian tern at , .Harrison (1988) p. 368.Olsen & Larsson (1995) p. 25. 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. The noddies (genus Anous) have unusual notched-wedge shaped tails, the longest tail feathers being the middle-outer, rather than the central or outermost.Harrison (1988) pp. 387–390.Hutton & Drummond (2011) p. 226. Although their legs are short, terns can run well. They rarely swim, despite having webbed feet, usually landing on water only to bathe.
The majority of sea terns have light grey or white body plumage as adults, with a black cap to the head. The legs and bill are various combinations of red, orange, yellow, or black depending on species. The pale plumage is conspicuous from a distance at sea, and may attract other birds to a good feeding area for these fish-eating species. When seen against the sky, the white underparts also help to hide the hunting bird from its intended prey. The Inca tern has mainly dark plumage, and three species that mainly eat insects, black tern, white-winged tern, and black-bellied tern, have black underparts in the breeding season. Three of the noddies (brown noddy, black noddy, and lesser noddy) have dark plumage with a pale head cap, while the other two noddies (blue noddy and grey noddy, both of which were formerly placed in the genus Procelsterna) have paler grey plumage. The reason for their dark plumage is unknown, but it has been suggested that in tropical areas, where food resources are scarce, the less conspicuous colouration makes it harder for other noddies to detect a feeding bird. 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.Olsen & Larsson (1995) pp. 6–9.
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.Hume (1993) pp. 68–75. Parents and chicks can locate one another by call,Burton (1985) p. 123. 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.Constantine (2006) pp. 73–77.
Taxonomy The bird order Charadriiformes contains 18 coastal seabird and wader families. Within the order, the terns form a lineage with the gulls, and, less closely, with the skimmers, skuas, and auks. Early authors such as Conrad Gessner, Francis Willughby, and William Turner did not clearly separate terns from gulls, but Linnaeus recognised the distinction in his 1758 Systema Naturae, placing the gulls in the genus Larus and the terns in Sterna. He gave Sterna the description rostrum subulatum, "awl-shaped bill", referring to the long, pointed bills typical of this group of birds, a feature that distinguishes them from the thicker-billed gulls.Linnaeus (1758) p. 84.Jobling (2010) p. 338.. 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.Gochfeld & Burger (1996) pp. 624–645.
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.
Research in 2007 had suggested that the noddies were not terns at all, but were basal to all the other genera in Laridae, 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; this has now been followed by the IOC World Bird List version 14.1 in 2024.
Etymology The word "stearn" was used for these birds in Old English as early as the eighth century, and appears in the poem The Seafarer, written in the ninth century or earlier. Variants such as "tearn" occurred by the eleventh century, although the older form lingered on in Norfolk dialect for several centuries.Hume (1993) pp. 12–13. As now, the term was used for the inland black tern as well as the marine species.Jobling (2010) p. 365.Lockwood (1984) pp. 147, 153. Some authorities consider "tearn" and similar forms to be variants of "stearn", while others derive the English words from Scandinavian equivalents such as Danish and Norwegian terne or Swedish tärna, and ultimately from Old Norse þerna.Merriam-Webster (2014) 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)Turner (1544), p. 78. or a North Germanic equivalent for his genus name Sterna.Linnaeus (1758) p. 155. All of these names are ultimately onomatopoeic, derived from the bird's calls.
Species The cladogram shows the relationships between the tern genera, and the currently recognised species, based on mitochondrial DNA studies, are listed below in the taxonomic sequence used by the Avilist.
Image GenusSpeciesGygis — noddies or white terns Atlantic white tern (Gygis alba) Blue-billed white tern (G. candida; recently split from G. alba) Little white tern (G. microrhyncha; recently split from G. alba)Anous — noddies Brown noddy (Anous stolidus) Lesser noddy (A. tenuirostris) Black noddy (A. minutus; sometimes considered to be a subspecies of lesser noddy) Grey noddy (A. albivitta; sometimes considered to be a subspecies of blue noddy; formerly placed in a separate genus Procelsterna) Blue noddy (A. cerulea; formerly placed in a separate genus Procelsterna)Onychoprion — brown-backed terns Aleutian tern (Onychoprion aleuticus) Sooty tern (O. fuscatus) Spectacled tern (O. lunatus) Bridled tern (O. anaethetus)Sternula — little terns Damara tern (Sternula balaenarum) Fairy tern (S. nereis) Little tern (S. albifrons) Yellow-billed tern (S. superciliaris) Peruvian tern (S. lorata) Saunders's tern (S. saundersi; formerly considered to be a subspecies of little tern)Harrison (1988) p. 382. Least tern (S. antillarum; formerly considered to be a subspecies of little tern)Phaetusa — large-billed tern Large-billed tern (Phaetusa simplex)Hydroprogne — Caspian tern Caspian tern (Hydroprogne caspia)Gelochelidon — gull-billed terns Australian tern (Gelochelidon macrotarsa) Gull-billed tern (G. nilotica)Larosterna — Inca tern Inca tern (Larosterna inca)Chlidonias — marsh terns Whiskered tern (Chlidonias hybridus) Black-fronted tern (C. albostriatus sometimes placed in Sterna) White-winged tern (or white-winged black tern; C. leucopterus) Black tern (C. niger)Thalasseus — crested terns Sandwich tern (T. sandvicensis) Cabot's tern (T. acuflavidus recently split from T. sandvicensis) Elegant tern (T. elegans) Chinese crested tern (T. bernsteini) Greater crested tern (T. bergii also known as swift tern) Royal tern (T. maximus) West African crested tern (Thalasseus albididorsalis; recently split from T. maximus) Lesser crested tern (Thalasseus bengalensis)Sterna — large white terns River tern (Sterna aurantia) Forster's tern (S. forsteri) Snowy-crowned tern (S. trudeaui) Arctic tern (S. paradisaea) South American tern (S. hirundinacea) Antarctic tern (S. vittata) Kerguelen tern (S. virgata) Common tern (S. hirundo) White-cheeked tern (S. repressa) Black-naped tern (S. sumatrana) Roseate tern (S. dougallii) White-fronted tern (S. striata) Black-bellied tern (S. acuticauda)
In addition to extant species, the fossil record includes a Miocene palaeospecies, Sterna milne-edwardsii.
The birds in the genus Anous are known as noddies, the Chlidonias species are the marsh terns, and all other species comprise the sea terns.Barlow et al. (1987) p. 212.Perrins & Arlott (1987) p. 130.
Distribution and habitat