Tern

Terns are seabirds in the family Laridae that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated as a subgroup of the family Laridae which includes gulls and skimmers and consist of eleven genera. 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 Inca tern, and some noddies have dark 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. The terns are birds of open habitats that typically breed in noisy colonies and lay their eggs on bare ground with little or no nest material. Marsh terns construct floating nests from the vegetation in their wetland habitats, and a few species build simple nests in trees, on cliffs or in crevices. The white tern, uniquely, lays its single egg on a bare tree branch. Depending on the species, one to three eggs make up the clutch. Most species feed on fish caught by diving from flight, but the marsh terns are insect-eaters, and some large terns will supplement their diet with small land vertebrates. Many terns are long-distance migrants, and the Arctic tern may see more daylight in a year than any other animal. 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 in a critical situation 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. The eggs of two species are eaten in the West Indies because they are believed to have aphrodisiac properties. The Charadriiformes order of birds 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. 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 1959, 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. 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. 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', while others derive the English words from Scandinavian equivalents such as Danish and Norwegian terne or Swedish tärna, and ultimately from Old Norse þerna. 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. The cladogram shows the relationships between the tern genera, and the currently recognised species, based on mitochondrial DNA studies, are listed below: In addition to extant species, the fossil record includes a Miocene palaeospecies, Sterna milne-edwardsii.