Sterna paradisaea (*)
Sterna paradisaea Pontoppidan, 1763
Den danske atlas eller konge-riget Dannemark 1 p.622
The Arctic Tern (Sterna paradisaea) is a seabird of the tern family Sternidae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America (as far south as Brittany and Massachusetts). The species is strongly migratory, seeing two summers each year as it migrates from its northern breeding grounds along a winding route to the oceans around Antarctica and back, a round trip of about 70,900 km (c. 44,300 miles) each year. This is by far the longest regular migration by any known animal. The Arctic Tern flies as well as glides through the air, performing almost all of its tasks in the air. It lands once every one to three years (depending on its mating cycle) to nest; once it has finished nesting it takes to the sky for another long southern migration.
Arctic Terns are medium-sized birds. They have a length of 33–39 cm (13–15 in) and a wingspan of 76–85 cm (26–30 in). They are mainly grey and white plumaged, with a red beak (as long as the head, straight, with pronounced gonys) and feet, white forehead, a black nape and crown (streaked white), and white cheeks. The grey mantle is 305 mm, and the scapulars are fringed brown, some tipped white. The upper wing is grey with a white leading edge, and the collar is completely white, as is the rump. The deeply forked tail is whitish, with grey outer webs. The hindcrown to the ear-coverts is black.
Arctic Terns are long-lived birds, with many reaching thirty years of age. They eat mainly fish and small marine invertebrates. The species is abundant, with an estimated one million individuals. While the trend in the number of individuals in the species as a whole is not known, exploitation in the past has reduced this bird's numbers in the southern reaches of its range.
The Arctic Tern has a worldwide, circumpolar breeding distribution which is continuous; 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. While wintering during the southern summer, it can be found at sea, reaching the southern edge of the Antarctic ice. The species' range encompasses an area of approximately ten million square kilometers.
The Arctic Tern is famous for its migration; it flies from its Arctic breeding grounds to the Antarctic and back again each year. This 19,000 km (12,000 mi) journey each way (measured point to point) ensures that this bird sees two summers per year and more daylight than any other creature on the planet. The average Arctic Tern in its lifetime of up to 34 years will travel about 2.4 million km (1.5 million mi). 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, which reached Melbourne, Australia, in October 1982, a sea journey of over 22,000 km (14,000 mi) in just three months from fledging. Another example is that of a chick ringed in Labrador, Canada, on 23 July 1928. It was found in South Africa four months later.
Research using tracking devices attached to the birds was published in January 2010 and showed that the above examples are in fact not unusual for the species; eleven Arctic Terns that bred in Greenland or Iceland each covered 70,900 km on average in a year, with a maximum of 81,600 km. The difference from previous estimates was because the birds were found to take a meandering course to take advantage of prevailing winds.
Arctic Terns usually migrate far offshore. Consequently, they are rarely seen from land outside the breeding season.
Description and taxonomy
The Arctic Tern is a medium-sized bird around 33–36 cm (13–15 in) from the tip of its beak to the tip of its tail. The wingspan is 76–85 cm. 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.
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.
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. The advertising call is social in nature, made when returning to the colony and during aggressive encounters between individuals. It is unique to each individual tern and as such it serves a similar role to the bird song of passerines, identifying individuals. Eight other calls have been described, from begging calls made by females during mating to attack calls made while swooping at intruders.
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.
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. On the wintering grounds, the Arctic Tern can be distinguished from these relatives; the six-month difference in moult is the best clue here, with Arctic Terns being in winter plumage during the southern summer. The southern species also do not show darker wingtips in flight.
When hatched, the chicks are downy. Neither altricial nor precocial, the chicks begin to move around and explore their surroundings within one to three days after hatching. Usually, they do not stray far from the nest. Chicks are brooded by the adults for the first ten days after hatching. Both parents care for hatchlings. Chick diets always include fish, and parents selectively bring larger prey items to chicks than they eat themselves. Males bring more food than females. Feeding by the parents lasts for roughly a month before being weaned off slowly. After fledging, the juveniles learn to feed themselves, including the difficult method of plunge-diving. They will fly south to winter with the help of their parents.
Arctic Terns are long-lived birds that spend considerable time raising only a few young, and are thus said to be K-selected. The maximum recorded life span for the species is 34 years. A life span of twenty years may not be unusual, with a study in the Farne Islands estimating an annual survival rate of 82%.
Ecology and behaviour
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.
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.
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. While feeding, skuas, gulls, and other tern species will often harass the birds and steal their food. They often form mixed colonies with other terns, such as Common and Sandwich Terns.
At the southern part of their range, the Arctic Tern has been declining in numbers. Much of this is due to shortages of food. However, most of these birds' range is extremely remote, with no apparent trend in the species as a whole.
Birdlife International has considered the species to be at lower risk since 1988, believing that there are approximately one million individuals around the world.
The Arctic Tern has appeared on the postage stamps of several countries and dependent territories. Territories include the Åland Islands, Alderney, and Faroe Islands. Countries include Canada, Finland, Iceland, and Cuba.
* Peter Harrison (1983). Seabirds. ISBN 0713646268
1. ^ BirdLife International (2004). Sterna paradisaea. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 11 May 2006. Database entry includes justification for why this species is of least concern
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License