Sterna artica

Tassonomia

The Arctic tern is a medium-sized bird around 33–36 cm (13–14 in) from the tip of its beak to the tip of its tail. The wingspan is 76–85 cm (30–33 in). The weight is 86–127 g (3.0–4.5 oz). The beak is dark red, as are the short legs and webbed feet. Like most terns, the Arctic tern has high aspect ratio wings and a tail with a deep fork.<p>The adult plumage is grey above, with a black nape and crown and white cheeks. The upperwings are pale grey, with the area near the wingtip being translucent. The tail is white, and the underparts pale grey. Both sexes are similar in appearance. The winter plumage is similar, but the crown is whiter and the bills are darker.<p>Juveniles differ from adults in their black bill and legs, "scaly" appearing wings, and mantle with dark feather tips, dark carpal wing bar, and short tail streamers. During their first summer, juveniles also have a whiter forecrown.<p>The species has a variety of calls; the two most common being the alarm call, made when possible predators (such as humans or other mammals) enter the colonies, and the advertising call.<p>While the Arctic tern is similar to the common and roseate terns, its colouring, profile, and call are slightly different. Compared to the common tern, it has a longer tail and mono-coloured bill, while the main differences from the roseate are its slightly darker colour and longer wings. The Arctic tern's call is more nasal and rasping than that of the common, and is easily distinguishable from that of the roseate.<p>This bird's closest relatives are a group of South Polar species, the South American (Sterna hirundinacea), Kerguelen (S. virgata), and Antarctic (S. vittata) terns.<p>The immature plumages of Arctic tern were originally described as separate species, Sterna portlandica and Sterna pikei.

Nome scientifico

Sterna paradisaea

Autorità

Pontoppidan, 1763

Ordine

Caradriformi

Famiglia

Laridae

Genere

Sterna

Areale e habitat

The Arctic tern has a continuous worldwide circumpolar breeding distribution; there are no recognized subspecies. It can be found in coastal regions in cooler temperate parts of North America and Eurasia during the northern summer. During the southern summer, it can be found at sea, reaching the northern edge of the Antarctic ice.<p>The Arctic tern is famous for its migration; it flies from its Arctic breeding grounds to the Antarctic and back again each year. The shortest distance between these areas is 19,000 km (12,000 mi). The long journey ensures that this bird sees two summers per year and more daylight than any other creature on the planet. One example of this bird's remarkable long-distance flying abilities involves an Arctic tern ringed as an unfledged chick on the Farne Islands, Northumberland, UK, in the northern summer of 1982 that reached Melbourne, Australia in October, just three months after fledging – a journey of more than 22,000 km (14,000 mi). Another example is that of a chick ringed in Labrador, Canada, on 23 July 1928. It was found in South Africa four months later.<p>A 2010 study using tracking devices attached to the birds showed that the above examples are not unusual for the species. In fact, the study showed that previous research had seriously underestimated the annual distances travelled by the Arctic tern. Eleven birds that bred in Greenland or Iceland covered 70,900 km (44,100 mi) on average in a year, with a maximum of 81,600 km (50,700 mi). The difference from previous estimates is due to the birds taking meandering courses rather than following a straight route as was previously assumed. The birds follow a somewhat convoluted course in order to take advantage of prevailing winds. The average Arctic tern lives about 30 years and will, based on the above research, travel some 2.4 million km (1.5 million mi) during its lifetime, the equivalent of a roundtrip from Earth to the Moon more than three times.<p>A 2013 tracking study of half a dozen Arctic terns breeding in the Netherlands shows average annual migrations of c. 48,700 km (30,300 mi). On their way south, these birds roughly followed the coastlines of Europe and Africa.<p>Arctic terns usually migrate sufficiently far offshore that they are rarely seen from land outside the breeding season.

Areale riproduttivo

PAL, NA : widespread Arctic and subarctic coasts and inland, from Greenland, Iceland and British Isles to ne Siberia, Sakhalin and Kamchatka (e Russia), Aleutian Is., Alaska mainland through Arctic and subarctic Canada to James Bay, se Canada and New York

Areale non riproduttivo

subantarctic and oceans and Antarctic Ocean to edge of pack-ice

Migrazione

Complete migrants

Comportamento

The diet of the Arctic tern varies depending on location and time, but is usually carnivorous. In most cases, it eats small fish or marine crustaceans. Fish species comprise the most important part of the diet, and account for more of the biomass consumed than any other food. Prey species are immature (1–2-year-old) shoaling species such as herring, cod, sandlances, and capelin. Among the marine crustaceans eaten are amphipods, crabs and krill. Sometimes, these birds also eat molluscs, marine worms, or berries, and on their northern breeding grounds, insects.<p>Arctic terns sometimes dip down to the surface of the water to catch prey close to the surface. They may also chase insects in the air when breeding. It is also thought that Arctic terns may, in spite of their small size, occasionally engage in kleptoparasitism by swooping at birds so as to startle them into releasing their catches. Several species are targeted—conspecifics, other terns (like the common tern), and some auk and grebe species.<p>While nesting, Arctic terns are vulnerable to predation by cats and other animals. Besides being a competitor for nesting sites, the larger herring gull steals eggs and hatchlings. Camouflaged eggs help prevent this, as do isolated nesting sites. Scientists have experimented with bamboo canes erected around tern nests. Although they found fewer predation attempts in the caned areas than in the control areas, canes did not reduce the probability of predation success per attempt. While feeding, skuas, gulls, and other tern species will often harass the birds and steal their food.

Conservazione

The total population for the arctic tern is estimated at more than two million individuals, with more than half of the population in Europe. The breeding range is very large, and although the population is considered to be decreasing, this species is evaluated as a species of least concern by the IUCN.Arctic terns are among the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds applies.<p>The population in New England was reduced in the late nineteenth-century because of hunting for the millinery trade. Exploitation continues in western Greenland, where the population of the species has been reduced greatly since 1950. In Iceland, the Arctic tern has been regionally uplisted to Vulnerable as of 2018, due to the crash of sandeel (Ammodytes spp.) stocks.<p>At the southern part of their range, the Arctic tern has been reducing in numbers. Much of this is due to a lack of food. However, most of these birds' range is extremely remote, with no apparent trend in the species as a whole. The Arctic terns' dispersal pattern is affected by changing climatic conditions, and its ability to feed in its Antarctic wintering is dependent on sea-ice cover, but unlike breeding species, it is able to move to a different area if necessary, and can be used as a control to investigate the effect of climate change on breeding species.

Tendenza della popolazione

declining