Animal kingdoms longest known migration revealed.

An Arctic tern at Sand Island, High-Arctic Greenland. Photographer: Carsten Egevang.

January 2010. The Arctic tern flies over 70,000 kilometres on its annual migration trip from pole to pole - the equivalent of three trips to the moon and back over its lifetime, according to an international team of scientists.

The study confirms what has been suspected for decades - the Arctic tern has the longest annual migration in the world. Scientists from British Antarctic Survey (BAS) worked with researchers from Greenland, Denmark, the United States and Iceland to successfully map the impressive migratory movements of the Arctic tern.

The Arctic terns were fitted with a tiny geolocator (1.4g); these geolocators regularly record light intensity, which can be used to generate two geographical positions per day.

The Arctic terns were fitted with a tiny geolocator (1.4g);

"The use of these devices on seabirds is not only revolutionising our understanding of migration patterns, but the resulting data on distribution also help address the requirement to identify important biological hotspots," said Richard Phillips from BAS, co-author of the paper.

Greenland to the Weddell Sea - 71,000 kilometre round trip
Every year the small seabird travels an average of around 71,000 km roundtrip from Greenland to the Weddell Sea, on the shores of Antarctica, and back to the breeding grounds in Greenland.

Stopover in the Azores
It turns out that the birds did not immediately travel south, but spent almost a month at sea in the middle of the North Atlantic Ocean, approximately 1,000 km north of the Azores. After this lengthy stop-over, the birds continued their long journey south down the coast of northwest Africa, but around the Cape Verde Islands the birds' behaviour surprised the research team again. Approximately half of the birds continued down the coast of Africa, while the other half crossed the Atlantic Ocean to follow a parallel route south down the east coast of South America.

Long-way home
All of the birds spent the northern winter months in Antarctic waters. Interestingly, on their long return journey the birds did not choose the shortest route back to their breeding grounds in Greenland. Instead, the Arctic terns traced out a gigantic ‘S' pattern northward through the Atlantic Ocean - a detour of several thousand kilometres over a straight line north to their breeding colonies.

Interpolated geolocation tracks of 11 Arctic terns tracked from breeding colonies in Greenland and Iceland. Green = autumn (postbreeding) migration (August-November) Red = winter range (December-March) Yellow = spring (return) migration (April-May). Dotted lines link locations during the equinoxes.

"This study on seabird migration has given us an incredibly detailed insight into how long-distance migrants behave at times of the year when it's normally impossible for us to follow them," said Carsten Egevang of the Greenland Institute of Natural Resources, lead author of the paper.

Carsten Egevang continued, "Our analysis shows that the birds' behaviour is closely correlated with both biological and physical parameters along the migration route. They paused in their southward migration to spend time in highly productive waters in the middle of the Atlantic Ocean. Compared to this stop-over site, the marine area immediately to the south is lower in productivity. Clearly, Arctic terns have learned to "fuel up" before crossing areas of ocean with limited foraging options."

Prevailing winds
"The indirect ‘S-shaped' return journey in spring indicates that Arctic terns take full advantage of the prevailing global wind systems to reduce energetic costs on their long flight north," said co-author, Iain Stenhouse.

Arctic terns can live up to 34 years
Arctic terns can live up to 34 years of age and they make the annual journey between the Arctic and Antarctic throughout their adult life. When added up, the total distance an Arctic tern flies over its lifetime is the equivalent of three journeys to the moon and back. "This is a mind-boggling achievement for a bird of just over 100 grams," concluded Carsten Egevang.

The simplified figure shows migration patterns of the Arctic tern, from the breeding sites in Greenland and Iceland to the winter grounds at Antarctica. After initiating the southbound migration (yellow line) the birds pause their migration in the central part of the North Atlantic (small circle) for almost a month before they continue towards the wintering sites at Antarctica (large circle). In spring, the northbound migration (white line) is conducted more than twice as fast in a gigantic "S" shaped pattern through the Atlantic Ocean. Areas particularly rich in biological productivity are indicated by yellow and green colours.

The study was published in U.S. journal Proceedings of the National Academy of Sciences

Simplified map showing migration patterns of the Arctic tern, from the breeding sites in Greenland and Iceland to the winter grounds at Antarctica. After initiating the southbound migration (yellow line) the birds paused their migration in the central part of the North Atlantic (small circle) for almost a month before they continue towards the wintering sites at Antarctica (large circle). In spring, the northbound migration (white line) is conducted more than twice as fast in a gigantic "S" shaped pattern through the Atlantic Ocean. Areas particular rich in biological productivity are indicated by yellow and green colours.