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Arctic tern

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Arctic Tern
Two Arctic Terns, one of which has just found a meal
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Aves
Order:
Charadriiformes
Family:
Sternidae
Genus:
Sterna
Species:
S. paradisaea
Binomial name
Sterna paradisaea
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 abundantly in Arctic and sub-Arctic regions of Europe, Asia, and North America (as far south as Brittany and Massachusetts). Arctic Terns are medium-sized birds, weighing between 86-127 grams and with a length of 33-39 centimetres and a wingspan of 66-77 cm.

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 eat mainly fish and small marine invertebrates. Like most terns, they plunge dive for food over the water.

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.[2] The area of the range of the species is approximately ten million square kilometers.[1]

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 mile) journey ensures that this bird sees two summers and more daylight than any other creature on the planet.[3] 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 miles) in just three months from fledging.[4] Another example is that of a chick ringed in Labrador on 23 July, 1928. It was found in South Africa four months later.[5] The average Arctic Tern in its life will travel a distance equal to going to the moon and back.[6]

Arctic Terns usually migrate far offshore.[7] Consequently, they are rarely seen from land outside the breeding season.

Physical description and taxonomy

An Arctic Tern in Finland

The Arctic Tern is medium-sized bird approximately 33-36cm (13-15in) from the tip of its beak to the tip of its tail. The wingspan is 76-85 cm.[7]

The beak is dark red. The legs are short, with three webbed toes, and are also dark red. The weight is 86-127 g. Like most terns, the Arctic Tern has high aspect ratio wings and a tail with a deep fork.[7]

The adult is grey above, with a black nape and white cheeks. The wings are grey from above. An area near the wingtip is translucent. The tail is white, and the underparts very pale grey. Both sexes are similar in appearance. Its upperwings are uniformly pale grey. It has a large area of translucent flight feathers on the wings. The winter plumage is similar, but the crown is whiter and the bills are darker.[7]

An Arctic Tern in flight with wings spread

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.[7].

The call is clear, harsh, nasal, and rasping.[8]

The Arctic Tern is very similar to the Common and Roseate Terns. It can be distinguished from the Common by its longer tail and mono-coloured bill. The main difference from the Roseate is its slightly darker colour and longer wings. The call is more nasal and rasping than that of the Common. Its call is very different from that of the Roseate.[8]

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.[9] On the wintering grounds, the Arctic Tern also has to be distinguished from its Antarctic 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.

Reproduction

A nesting Arctic Tern at Farne Islands, Northumberland, England.

Arctic terns mate for life.[10] In most cases, they return to the same colony each year. Breeding begins around the third or fourth year.[11] Courtship is elaborate, especially in birds nesting for the first time.[12] Courtship begins with a so-called "high flight", where a female will chase the male to altitude, followed by a slow descent. This display is followed "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.[12]

Both sexes agree on a site for a nest. When both birds are present, both will defend the site. During this time, there is additional feeding of the female by the male. Mating occurs shortly after this.[12] 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.[7]

It is one of the most aggressive terns, fiercely defensive of its nest and young. It will attack humans and other 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.[3] In this it differs from the Common Tern, which usually veers off at the last moment, relying on bluff to deter predators without actually striking. 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.[7] Both sexes share incubation duties. The young hatch after 22-27 days and fledge after 21-24 days.[7] 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.[13] Both parents care for the hatchlings and bring them food for roughly a month.[7]

When hatched, the chicks are downy. They are moderately altricial; beginning to move around and explore their surroundings within one to three days after hatching.[14] Usually, they do not stray for from the nest. Chick diets always include fish, and parents selectively bring larger prey items to chicks than they eat themselves.[13] Males bring more food than females. Feeding by the parents lasts for a considerable time before being weaned off slowly. After fledging, the juveniles learn to feed themselves, including the difficult method of plunge-diving.[15] They will fly south to winter with the help of their parents.[16]

Arctic Terns are K-selected; 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.[17] A lifespan of twenty years may not be unusual,[10] with a study in the Farne Islands estimating an annual survival rate of 82%.[18]

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.[2][7] 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.[19] Among the marine crustaceans eaten amphipods are the most commonly taken. Crabs and krill are also sometimes taken. Sometimes, these birds also eat molluscs, marine worms, or even berries, and on their northern breeding grounds, insects.[14]

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 a "stepped-hover", an Arctic Tern will slowly fly 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.[14] This way of feeding is quite difficult.[15]

Arctic Terns sometimes also simply 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.[14] It is also thought that Arctic Terns may, in spite of their small size, on rare occasions, engage in kleptoparasitism by swooping at birds so as to startle them into releasing their catches.[14]

While nesting, Arctic Terns are vulnerable to predation by rats and other animals.[2] The isolated nesting sites in remote areas and camouflaged eggs are attempts to get around this. Besides stealing eggs and hatchlings, the larger Herring Gull is a major competitor for nesting sites.[15] While feeding, skuas will often harass the birds and steal their food.[20]

Conservation Status

An Arctic Tern up close

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.[21] The species declined in New England in the late nineteenth-century due to hunting for the millinery trade.[13] Exploitation continues today in western Greenland, where the species has declined greatly since 1950.[22]

At the southern part of their range, the Arctic Tern has been declining in numbers. However, most of these birds' range is extremely remote, with no apparant trend in the species as a whole.[14]

Birdlife International considers this species to be lower risk, which it has done so since 1988. While the population trend as a whole is not known, it is believed that there are roughly one million individuals.[1]

Appearances on stamps

The Arctic Tern has appeared on the postage stamps of several countries and dependant territories. Territories include the Aland Islands, Faeroe Islands, and Alderney. Countries include Canada, Finland, Iceland, even Cuba.[23]

Notes and references

  1. ^ a b c Birdlife International. "Arctic Tern - BirdLife Species Factsheet". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  2. ^ a b c Royal Society for the Preservation of Birds. "Arctic tern". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  3. ^ a b S. Cramp, ed. (1985). Birds of the Western Palearctic. pp. 87–100. ISBN 0-19-857507-6.
  4. ^ A. Heavisides (1983). Birds in Northumbria 1982. Tyneside Bird Club. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  5. ^ "Birds of Nova Scotia:Arctic Tern". Nova Scotia Museum of Natural History. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  6. ^ British Trust for Ornithology. "Terns (Family Sternidae)". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  7. ^ a b c d e f g h i j Josep del Hoyo (ed.). Handbook of the Birds of the World vol. 3. Lynx Edicions. p. 653. ISBN 84-87334-20-2. {{cite book}}: Unknown parameter |coeditors= ignored (help)
  8. ^ a b Klaus Malling Olson (1995). Terns of Europe and North America. Princeton University Press. ISBN 0-7136-4056-1. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ E.S. Bridge. "A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution" (PDF). Molecular phylogenetics and Evolution 35. pp. 459–69. {{cite web}}: Italic or bold markup not allowed in: |work= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ a b Christopher Perrins (ed.). Firefly Encyclopedia of Birds. Firefly Books. p. 267. ISBN 0-307-13656-6.
  11. ^ Oscar Hawksley (1957). "Ecology of a breeding population of Arctic Terns" (PDF). Bird-Banding 28. pp. 57–92. {{cite web}}: Italic or bold markup not allowed in: |work= (help); Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  12. ^ a b c Perrins p. 268
  13. ^ a b c J.J. Hatch (2002). A. Poole; & F. Gill (ed.). Arctic Tern (Sterna paradisaea). Philadelphia, PA.: The Birds of North America. p. 707. {{cite book}}: |work= ignored (help)CS1 maint: multiple names: editors list (link)
  14. ^ a b c d e f Ken Kaufman. Lives of North American birds. Houghton Mifflin. p. 260. ISBN 0-395-77017-3.
  15. ^ a b c Perrins p. 269
  16. ^ National Audubon Society. "Arctic Tern (Sterna paradisaea)". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  17. ^ Jeremy J. Hatch (1974). "Longevity record for the Arctic Tern" (PDF). Bird-Banding Volume 45. pp. 269–270. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  18. ^ J.M. Cullen (1957). Plumage, age and mortality in the Arctic Tern. Vol. 4. pp. 197–207. {{cite book}}: |work= ignored (help)
  19. ^ S. Cramp, ed. (1985). The birds of the western Palearctic. Vol. 4: terns to woodpeckers. Oxford, UK: Oxford Univ. Press. ISBN 0-19-857507-6.
  20. ^ Perrins p. 271
  21. ^ AEWA. "African Eurasian Waterbird Agreement Annex II: Species list". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  22. ^ K. Hansen (2001). Threats to wildlife in Greenland. pp. 1–2. {{cite book}}: |work= ignored (help)
  23. ^ Chris Gibbons. "Arctic Tern stamps". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  • Template:IUCN2006 Database entry includes justification for why this species is of least concern
  • Peter Harrison (1983). Seabirds. ISBN 0-7470-8028-8.
  • National Geographic (3rd ed., 1999). Field Guide to the Birds of North America. ISBN 0-7922-6877-6.
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