Arctic tern
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Arctic tern ( Sterna paradisaea ) |
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| Sterna paradisaea | ||||||||||
| Pontoppidan , 1763 |
The arctic tern ( Sterna paradisaea ) is a species of bird from the family of the tern (Sternidae). It is considered to be the migratory bird with the longest migration distance, as it breeds in the north polar region and winters in the south polar regions.
description
The arctic tern reaches a body size between 33 and 36 cm. The wing length is 23.9 to 29 cm and the wingspan 76 to 85 cm. The weight varies between 86 and 127 grams.
The arctic tern is very similar to the common tern ( St. hirundo ). Their plumage is white to light gray in color. In its splendid plumage, the arctic tern's beak is colored uniformly red, without a distinct black tip section as in the common tern. The black head cap only extends far into the neck of the common tern, while it is shorter in the arctic tern. In the plain dress, the beak turns black and the top of the head white. The arctic tern has very short legs compared to the common tern, but its tail skewers are all the longer. But even with the help of these characteristics, it is difficult to distinguish the two types.
In their youthful dress, arctic terns do not yet have a gray belly, the cap does not yet reach the forehead, and the back is sometimes still a bit brownish or streaky.
Your reputation sounds like "crazy".
habitat
As a thrust diver, the arctic tern usually prefers clear coastal stretches with little vegetation. Similar to the common tern, with which there is some overlap in the use of the biotope , the arctic tern seeks the large estuaries and the spacious tidal areas between the mainland and the islands. The rich structuring of the tidal flats , with its branching system of tidal creeks and baljas , ditches, groynes and tide pools, seems to be important for both . In contrast to the common tern, however, the arctic tern's range of action when foraging is much smaller.
distribution
Arctic terns breed in a circumpolar holoarctic manner from the boreal zones to the high arctic , only with gaps in their distribution due to summer ice. In Central Europe , it has its southernmost distribution area as a regular breeding bird on the North and Baltic Sea coasts . Here, during the breeding season, it can only be found in small numbers on the East Frisian Islands and in somewhat larger numbers on Scharhörn . Larger numbers breed on the North Sea coast of Schleswig-Holstein and Denmark . At the Baltic Sea it is a regular breeding bird in the Wismar Bay . In the Red List of Germany's breeding birds from 2015, the species is listed in Category 1 as critically endangered.
food
Arctic terns eat small fish , insects , pelagic and littoral crustaceans . In so-called shock diving, the birds first carefully search the water surface in a slow search flight. If the arctic tern has discovered a prey, it suddenly tips vertically with half-closed wings and thrusts downwards at a steep angle. The bird disappears completely in the water when it is immersed, but after a short time it comes out again with its wings first.
migration
The arctic tern is the migratory bird with the longest migration route because it likes the sun as a day hunter. Their winter quarters are on the edge of the Antarctic pack ice zone between the 55th and 70th parallel south of the Atlantic and Indian Ocean , in the Weddell Sea and off the Antarctic . On their migration from the Arctic breeding sites to the Antarctic wintering areas and back, the birds cover a distance of up to 30,000 km - almost once around the world. Recent research has shown that individual individuals can travel up to 90,000 km in a year. One of 29 sensor-equipped animals from the Farne Islands traveled 96,000 km in 10 months.
Both habitats, i.e. the northern and southern polar regions , offer an abundance of food for the birds in the summer months. By exploiting the polar summers of the Arctic and Antarctic, the arctic terns that hunt visually have the additional advantage that the sun does not set in their habitat for a total of eight months and they can (theoretically) search for food 24 hours a day (they can go for a maximum of approx. Fly 520 km a day).
Which route the arctic tern takes on its way between the two "poles" depends on the individual breeding place of the animals. Birds that breed in the high latitudes of eastern North America cross the Atlantic and meet arctic terns from northwestern Europe near the Iberian Peninsula . Together they fly south to the western tip of Africa . At Cape Verde , the group splits into three migration routes. One group follows the African coast, a second turns directly over the open South Atlantic towards Antarctica, and a third crosses the Atlantic to South America at this altitude and follows the coast to Cape Horn. The smaller sub-populations from western North America and eastern Asia meet at the beginning of their journey and follow the west coast of North and South America to move in a south-easterly direction to the southern tip of South America. All four groups put the last and most strenuous stage back together in their main wintering quarters, which is in the south of the Indian Ocean. The migration route of the arctic tern is primarily based on the prevailing wind directions, which the animals use and thus increase their average speed by up to 30 km / h. Arctic terns resident in Greenland can cover an average of around 330 km per day on the outward journey towards the Weddell Sea . On the return trip they can even manage up to 520 km a day: because of the more favorable wind systems , they choose an S-shaped route through the middle of the South Atlantic .
The arctic tern overcomes the long exertions of migration by means of a "sleep on the fly": While one half of the brain is sleeping, the other is navigating and controlling the flight.
As they migrate home, the birds can take advantage of the east wind near the mainland caused by the receding pack ice. Arctic terns, trapped in the westerly wind zone, drifting too far to the east can only begin their northward migration home after they have circled Antarctica.
Mauser
In adaptation to their extreme migration path, the arctic tern deviates greatly from the moulting rhythm of other terns. In contrast to the common tern, all wings and control springs are only changed once. This rocking moult occurs over a period of 3 to 3½ months, exclusively in the pack ice of the Arctic. In the common tern, it takes about ten months to renew the wings of the hand.
Reproduction
The birds reach sexual maturity at three to five years, with the majority in the fourth year for the first time brooding . Like the seagulls, these terns breed in colonies of up to 1000 animals. The preferred breeding habitat are islands with little vegetation , in sand dunes and gravel beds. The nest is a hollow turned into the ground with the bird's body, which is usually only poorly lined with stalks . The female lays one to three 4 cm eggs . Both parents warm the eggs for about three weeks until the chicks hatch. The young fly out at 21 to 24 days and are usually fed for another week. The brood is defended against enemies by diving.
Breeding success and age
The hunting technique of the terns, which is not innate but has to be learned, leads to high losses in the young birds. With the technique of shock diving it is difficult for the animals to take into account the error that occurs due to the refraction of light on the water surface. The arctic tern has a life expectancy of around ten to eleven years. The oldest bird so far was checked in a breeding colony at the age of 34 (as an active breeding bird).
Stock size and stock development
The arctic tern reaches its regional southern border on the Central European coasts. In addition to the Central European breeding population of 6,000 to 7,000 pairs, there is only one irregularly occupied breeding site in Brittany. Overall, the information on the population figures is uncertain, since the coastal tern and common tern have often been added together.
literature
- Hadoram Shirihai: A Complete Guide to Antarctic Wildlife - The Birds and Marine Mammals of the Antarctic Continent and Southern Ocean , Alula Press, Degerby 2002, ISBN 951-98947-0-5
Web links
- various terns photographed in the wild.
- Sterna paradisaea in the endangered Red List species the IUCN 2008. Posted by: BirdLife International, 2008. Accessed January 30 of 2009.
- Videos, photos and sound recordings of Sterna paradisaea in the Internet Bird Collection
- Distinction between common and arctic tern with comments on the forest and rose tern, technical article by Peter H. Barthel from Limicola, Volume 5 (1991), page 1 (PDF; 6.0 MB)
- Arctic tern feathers
Individual evidence
- ^ Shirihai, p. 232
- ↑ Christoph Grüneberg, Hans-Günther Bauer, Heiko Haupt, Ommo Hüppop, Torsten Ryslavy, Peter Südbeck: Red List of Germany's Breeding Birds , 5 version . In: German Council for Bird Protection (Hrsg.): Reports on bird protection . tape 52 , November 30, 2015.
- ↑ a b Carsten Egevang, Iain J. Stenhouse, Richard A. Phillips, Aevar Petersen, James W. Fox, and Janet RD Silk: Tracking of Arctic terns Sterna paradisaea Reveals longest animal migration . In: PNAS Early Edition . 107, No. 5, January 2010, pp. 2078-2081. doi : 10.1073 / pnas.0909493107 . PMID 20080662 .
- ↑ RC Fijn, D. Hiemstra, RA Phillips, J. van der Winden: Arctic Terns Sterna paradisaea from the Netherlands migrate record distances across three oceans to Wilkes Land, East Antarctica . ARDEA 101 (2013) 3-12 .
- ↑ English tern flies world record , NZZ, June 8, 2016
- ↑ Farne Islands Uncovered: Amazing Arctic Terns. In: farnephoto.blogspot.de. Retrieved September 30, 2016 .