Image: Arctic tern (Sterna paradisaea)
Carsten Egevang
The Arctic tern is an elegant flyer performing an annual long-distance migration between the Arctic and the Antarctica. When added up over a lifetime, the total journey for the bird is the equivalent of three trips to the moon and back.
updated 1/11/2010 6:29:59 PM ET 2010-01-11T23:29:59

Some birds make long treks south during wintertime, but the Arctic tern bests them all, flying on average 44,000 miles on its annual migration from pole to pole, according to a new study.

The shortest journey recorded for the tern was 36,900 miles and the longest was 50,700 miles.

The study confirms what has been supposed for decades — the Arctic tern has the longest annual migration of any animal in the world. When added up over a lifetime, the total journey for the bird is the equivalent of three trips to the moon and back.

The researchers used a tiny instrument, called a geolocator, for tracking Arctic tern migration. The device regularly records light intensity, which can be used to generate two geographical positions per day.

While geolocators have helped scientists follow the journeys of larger animals, including geese, albatross, penguins and seals, they have only recently been made small enough to attach to lightweight birds, such as the tern. The researchers fitted geolocators weighing about 1.4 grams each to the legs of 60 terns.

The study showed that the Arctic tern travels an average of around 44,000 miles round trip from Greenland to the Weddell Sea, on the shores of Antarctica, and back to the breeding grounds in Greenland — nearly twice the distance generally cited for tern's annual migration, the researchers say.

A month at sea
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. The researchers believe the birds use this lengthy stop-over as a chance to "fuel up" with food before continuing on to less fruitful waters farther south.

After this pause, the birds continued their long journey south down the coast of northwest Africa. However, around the Cape Verde Islands — islands off the west coast of Africa, close to Senegal — the birds' behavior surprised the research team again. About 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.

All of the birds spent the northern winter months in Antarctic waters.

Following an 'S' pattern
Interestingly, on their return journey the birds did not choose the shortest route back to their breeding grounds in Greenland. Instead, they traced out a gigantic "S" pattern northward through the Atlantic Ocean — a detour of several thousand kilometers compared to a straight line north to their breeding colonies.

This indirect route allows the terns to take advantage of the global wind system and to reduce the amount of energy it needs to use on its return journey, said Iain Stenhouse, a co-author of the research paper, published Monday in the Proceedings of the National Academy of Sciences.

"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.

The study was conducted by scientists from the British Antarctic Survey in collaboration with researchers from Greenland, Denmark, the United States and Iceland.

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