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Arctic Tern

2007 Schools Wikipedia Selection. Related subjects: Birds

                               iArctic Tern
   Two Arctic Terns, one of which has just found a meal
   Two Arctic Terns, one of which has just found a meal

                             Conservation status

   Least Concern (LC)
                         Scientific classification

   Kingdom: Animalia
   Phylum:  Chordata
   Class:   Aves
   Order:   Charadriiformes
   Family:  Sternidae
   Genus:   Sterna
   Species: S. paradisaea

                                Binomial name

   Sterna paradisaea
   Pontoppidan, 1763
   Breeding grounds (red), wintering grounds (blue) and migration routes
   (green)
   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.

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. The area of the range of the species is 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 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 life will travel a distance equal to going
   to the moon and back. 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.
   Another example is that of a chick ringed in Labrador on 23 July 1928.
   It was found in South Africa four months later.

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

Physical description and taxonomy

   An Arctic Tern in Finland
   Enlarge
   An Arctic Tern in Finland

   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. 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.
   An Arctic Tern in flight with wings spread
   Enlarge
   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. During their first summer, juveniles
   also have a whiter forecrown.

   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.

Reproduction

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

   Arctic Terns mate for life, and in most cases, return to the same
   colony each year. Breeding begins around the third or fourth year.
   Courtship is elaborate, especially in birds nesting for the first time.
   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.

   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. 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.

   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. 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. Both sexes share incubation duties. The young hatch after
   22-27 days and fledge after 21-24 days. 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.

   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 trend towards K-selection; 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. A lifespan 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.

   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.

   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 rats 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.

Conservation status

   An Arctic Tern up close
   Enlarge
   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. The
   species declined in New England in the late nineteenth-century due to
   hunting for the millinery trade. Exploitation continues today in
   western Greenland, where the species has declined greatly since 1950.

   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.

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.
   Retrieved from " http://en.wikipedia.org/wiki/Arctic_Tern"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
