Arctic Tern
From Wikipedia, the free encyclopedia
| iArctic Tern | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Two Arctic Terns, one of which has just found a meal
| ||||||||||||||
| | ||||||||||||||
|
</div> | ||||||||||||||
| Scientific classification | ||||||||||||||
| ||||||||||||||
| | ||||||||||||||
| Sterna paradisaea Pontoppidan, 1763<ref name="Birdlife International">Birdlife International. Arctic Tern - BirdLife Species Factsheet (English). Retrieved on 17 August, 2006.</ref> | ||||||||||||||
| Image:Sterna paradisaea distr mig.png Breeding grounds (red), wintering grounds (blue) and migration routes (green)
|
The Arctic Tern (Sterna paradisaea) is a seabird of the tern family Sternidae. This bird has a circumpolar distribution, breeding colonially in 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 to the oceans around Antarctica and back each year. This is the longest regular migration by any known animal.
Arctic Terns are medium-sized birds, with a length of 33-39 centimetres (13-15 in) and a wingspan of 76-85 cm (26-30 in). They are mainly grey and white plumaged, with a red beak and feet, a black-nape and crown, and white cheeks.
The Arctic Tern is K-selected, caring for and aggressively defending a small number of young. Parents feed them fish for a considerable time, and help them fly south to winter.
Arctic Terns are long-lived birds, with many reaching twenty 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.
Contents |
[edit] Distribution and migration
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.<ref name="RSPB">Royal Society for the Preservation of Birds. Arctic tern (English). Retrieved on 17 August, 2006.</ref> The area of the range of the species is approximately ten million square kilometers.<ref name="Birdlife International"/>
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 ensures that this bird sees two summers per year and more daylight than any other creature on the planet.<ref name=Cramp> (1985) S. Cramp: Birds of the Western Palearctic, 87-100. ISBN 0-19-857507-6.</ref> The average Arctic Tern in its life will travel a distance equal to going to the moon and back.<ref name="BTO">British Trust for Ornithology. Terns (Family Sternidae). Retrieved on 31 August, 2006.</ref> 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 summer 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.<ref name="Heavisides et. al">A. Heavisides, M.S. Hodgson & I Kerr (1983). Birds in Northumbria 1982. Tyneside Bird Club.</ref> Another example is that of a chick ringed in Labrador on 23 July, 1928. It was found in South Africa four months later.<ref name="Birds of Nova Scotia">Birds of Nova Scotia:Arctic Tern. Nova Scotia Museum of Natural History. Retrieved on 22 August, 2006.</ref>
Arctic Terns usually migrate far offshore.<ref name=HBW> [1996] Josep del Hoyo: Handbook of the Birds of the World vol. 3. Lynx Edicions, 653. ISBN 84-87334-20-2.</ref> Consequently, they are rarely seen from land outside the breeding season.
[edit] Physical description and taxonomy
The Arctic Tern is medium-sized bird approximately 33-36 cm (13-15 in) from the tip of its beak to the tip of its tail. The wingspan is 76-85 cm.<ref name=HBW/> 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.<ref name=HBW/>
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.<ref name=HBW/>
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.<ref name=HBW/> During their first summer, juveniles also have a whiter forecrown.<ref name="National Geographic">Steve N.G. Howell, & Alvaro Jaramillo (2006). Jonathan Alderfer: National Geographic Complete Birds of North America. National Geographic Society, 272-73. ISBN 0-7922-4175-4.</ref>
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.<ref name="Hatch et. al">J.J. Hatch (2002). A. Poole; & F. Gill: Arctic Tern (Sterna paradisaea). Philadelphia, PA.: The Birds of North America, 707.</ref> 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.<ref name=Olsen>Klaus Malling Olson, Hans Larsson (1995). Terns of Europe and North America. Princeton University Press. ISBN 0-7136-4056-1.</ref>
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.<ref name="Bridge et. al"> E.S. Bridge; A.W. Jones; & A.J. Baker. A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution. Molecular phylogenetics and Evolution 35 pp. 459-69. Retrieved on September 7, 2006.</ref> 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.
[edit] Reproduction
Arctic Terns mate for life, and in most cases, return to the same colony each year.<ref name="Firefly p 267"> [2003] Christopher Perrins: Firefly Encyclopedia of Birds. Firefly Books, 267. ISBN 0-307-13656-6.</ref> Breeding begins around the third or fourth year.<ref name=Hawksley>Oscar Hawksley (1957). Ecology of a breeding population of Arctic Terns. Bird-Banding 28 pp. 57–92. Retrieved on September 1, 2006.</ref> Courtship is elaborate, especially in birds nesting for the first time.<ref name="Firefly p 268">Perrins p. 268</ref> Courtship begins with a so-called "high flight", where a female will chase the male to a high altitude and then slowly descend. This display is followed by "fish flights", where the male will offer fish to the female. Courtship on the ground involves strutting with a raised tail and lowered wings. After this, both birds will usually fly and circle each other.<ref name="Firefly p 268"/>
Both sexes agree on a site for a nest, and both will defend the site. During this time, the male continues to feed the female. Mating occurs shortly after this.<ref name="Firefly p 268"/> Breeding takes place in colonies on coasts, islands and occasionally inland on tundra near water. It often forms mixed flocks with the Common Tern. It lays from one to three eggs per clutch, most often two.<ref name=HBW/>
It is one of the most aggressive terns, fiercely defensive of its nest and young. It will attack humans and large predators, usually striking the top or back of the head. Although it is too small to cause serious injury, it is capable of drawing blood.<ref name=Cramp/> Other birds can benefit from nesting in an area defended by Arctic Terns.
The nest is usually a depression in the ground, which may or may not be lined with bits of grass or similar materials. The eggs are mottled and camouflaged.<ref name=HBW/> Both sexes share incubation duties. The young hatch after 22-27 days and fledge after 21-24 days.<ref name=HBW/> If the parents are disturbed and flush from the nest frequently this has the effect of extending the incubation period to as long as 34 days.<ref name="Hatch et. al">J.J. Hatch (2002). A. Poole; & F. Gill: Arctic Tern (Sterna paradisaea). Philadelphia, PA.: The Birds of North America, 707.</ref>
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.<ref name=Kaufman>Ken Kaufman [1996]. Lives of North American birds. Houghton Mifflin, 260. ISBN 0-395-77017-3.</ref> Usually, they do not stray far from the nest. Chicks are brooded by the adults for the first ten days after hatching.<ref>Klaassen, M; Bech, C; Masman, D; Slagsvold, G (1989). Growth and energetics of Arctic tern chicks (Sterna paradisaea). Auk 106 pp. 240-248. Retrieved on September 1, 2006.</ref> Both parents care for hatchlings.<ref name=HBW/> Chick diets always include fish, and parents selectively bring larger prey items to chicks than they eat themselves.<ref name="Hatch et. al"/> Males bring more food than females. Feeding by the parents lasts for roughly a month before being weaned off slowly.<ref name=HBW/> After fledging, the juveniles learn to feed themselves, including the difficult method of plunge-diving.<ref name="Firefly p 269">Perrins p. 269</ref> They will fly south to winter with the help of their parents.<ref name=Audubon>National Audubon Society. Arctic Tern (Sterna paradisaea). Retrieved on 1 September, 2006.</ref>
Arctic Terns trend towards K-selection;<ref name=Burger>Elizabeth A. Schreiber, Joanne Burger (2001). Biology of Marine Birds. CRC Press. ISBN 0-8493-9882-7.</ref> they are long-lived birds with few young raised each year compared to many land birds. The maximum recorded lifespan for the species is 34 years.<ref name=Hatch">Jeremy J. Hatch (1974). Longevity record for the Arctic Tern. Bird-Banding Volume 45 pp. 269–270. Retrieved on September 7, 2006.</ref> A lifespan of twenty years may not be unusual,<ref name="Firefly p 267"/> with a study in the Farne Islands estimating an annual survival rate of 82%.<ref name=Cullen>J.M. Cullen (1957). Plumage, age and mortality in the Arctic Tern, 197–207.</ref>
[edit] 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.<ref name="RSPB"/><ref name=HBW/> 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.<ref name=Cramp/> 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.<ref name=Kaufman/>
Like all Sterna terns, the Arctic Tern feeds by plunge-diving for fish, usually from the sea, though occasionally also fishing in coastal freshwater lagoons. It often dives from a "stepped-hover", in which it slowly flies into the wind at such a speed that it is stationary relative to the water. It will then dive and catch its prey, which it detects by sight.<ref name=Kaufman/>
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.<ref name=Kaufman/> 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.<ref name=Kaufman/> Several species are targeted—conspecifics, other terns (like the Common Tern), and some auk and grebe species.<ref name="Hatch et. al"/>
While nesting, Arctic Terns are vulnerable to predation by rats and other animals.<ref name=RSPB/> 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.<ref name="Firefly p 269"/> While feeding, skuas, gulls, and other tern species will often harass the birds and steal their food.<ref name="Firefly p 271">Perrins p. 271</ref> They often form mixed colonies with other terns, such as Common and Sandwich Terns.
[edit] Conservation status
Arctic Terns are considered threatened or species of concern in certain states. They are also among the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds applies.<ref name=AEWA>AEWA. African Eurasian Waterbird Agreement Annex II: Species list (English). Retrieved on August 17, 2006.</ref> The species declined in New England in the late nineteenth-century due to hunting for the millinery trade.<ref name="Hatch et. al"/> Exploitation continues today in western Greenland, where the species has declined greatly since 1950.<ref name=Hansen>K. Hansen (2001). Threats to wildlife in Greenland, 1–2.</ref>
At the southern part of their range, the Arctic Tern has been declining in numbers. Much of this is due to shortages of food.<ref name="National Geographic"/> However, most of these birds' range is extremely remote, with no apparent trend in the species as a whole.<ref name=Kaufman/>
Birdlife International has considered the species to be at lower risk since 1988, believing that there are approximately one million individuals around the world.<ref name="Birdlife International"/>
[edit] Appearances on stamps
The Arctic Tern has appeared on the postage stamps of several countries and dependent territories. Territories include the Aland Islands, Alderney, and Faeroe Islands. Countries include Canada, Finland, Iceland, and Cuba.<ref name="Bird Stamps">Chris Gibbons. Arctic Tern stamps. Retrieved on 24 August, 2006.</ref>
[edit] Notes and references
- BirdLife International (2004). Sterna paradisaea. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Database entry includes justification for why this species is of least concern
- Peter Harrison (1983). Seabirds. ISBN 0-7470-8028-8.
[edit] External links
- Arctic Tern videos on the Internet Bird Collection
bg:Полярна рибарка cs:Rybák dlouhoocasý cy:Morwennol y Gogledd da:Havterne de:Küstenseeschwalbe et:Randtiir fr:Sterne arctique is:Kría it:Sterna paradisaea lt:Poliarinė žuvėdra nl:Noordse stern ja:キョクアジサシ no:Rødnebbterne pl:Rybitwa popielata fi:Lapintiira sv:Silvertärna
