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 along a convoluted route from its northern breeding grounds to the antarctic coast for the southern summer and back again about six months later .
recent studies have shown average annual roundtrip lengths of about 70,900 km (44,100 mi) for birds nesting in iceland and greenland and c .
90,000 km (56,000 mi) for birds nesting in the netherlands .
these are by far the longest migrations known in the animal kingdom .
the arctic tern flies as well as glides through the air , performing almost all of its tasks in the air .
it nests once every one to three years (depending on its mating cycle); 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 2839 cm (1115 in) and a wingspan of 6575 cm (2630 in) .
they are mainly grey and white plumaged , with a red/orangish beak and feet , white forehead , a black nape and crown (streaked white), and white cheeks .
the grey mantle is 305 mm , and the scapulae 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 .
arctic terns are long-lived birds , with many reaching fifteen to 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 was known as sea swallow describing their slender shape as they swoop over the water .
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 .
while wintering during the southern summer , it can be found at sea , reaching the northern edge of the antarctic ice .
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 being 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, which in october 1982, just three months from fledging , reached melbourne , australia .
assuming a direct route of flight , the distance covered would have been 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 .
a 2010 study using tracking devices attached to the birds showed that the above examples are not unusual for the species .
in fact , it turned out , 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 thirty 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 over 3 times .
a 2013 tracking study of half a dozen arctic terns breeding in the netherlands shows average annual migrations of c .
90,000 km (56,000 mi) .
on their way south , these birds roughly followed the coastlines of europe and africa .
having rounded the southern tip of africa , they then turned east , some flying approximately halfway to australia before again turning south to eventually reach wilkes land in the north-eastern antarctic .
one bird flew several hundred kilometres along the south coast of australia before turning south for the antarctic , while one flew along the entire south coast of australia , passing between australia and tasmania .
having reached the melbourne area , it turned south and flew in an arc to wilkes land in the north-east antarctic , passing the south-western tip of new zealand's south island en route .
once back in the netherlands , this bird had journeyed c .
91,000 km (57,000 mi), the longest migration yet recorded for any animal .
arctic terns usually migrate sufficiently far offshore that they are rarely seen from land outside the breeding season .
the arctic tern is a medium-sized bird around 3336 cm (1314 in) from the tip of its beak to the tip of its tail .
the wingspan is 7685 cm (3033 in) .
the weight is 86127 g (3.04.5 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 spreadjuveniles 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 .
the immature plumages of arctic tern were originally described as separate species , sterna portlandica and sterna pikei .
breeding begins around the third or fourth year .
arctic terns mate for life , and in most cases , return to the same colony each 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 still 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 2227 days and fledge after 2124 days .
if the parents are disturbed and flush from the nest frequently the incubation period could be extended 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 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 , although the average lifespan is about 20 years a study in the farne islands estimated an annual survival rate of 82% .
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 (12 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 targetedconspecifics , 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 .
arctic terns are considered threatened or species of concern in certain states .
they are also among a species to which the agreement on the conservation of african-eurasian migratory waterbirds applies .
the species reduced population in new england in the late nineteenth-century because of hunting for the millinery trade .
exploitation continues today in western greenland , where the species has reduced population greatly since 1950.at the southern part of their range , the arctic tern has been reducing in numbers .
much of this is due to lack 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" .
