Pole to pole, the extraordinary migration of the Arctic tern

Bird travels equivalent of three round trips to the Moon in its lifetime

A sea bird famous for its long-distance migrations between the North and South Poles makes the equivalent of three round trips to the Moon in its lifetime, scientists have found.

The Arctic tern makes a return trip of around 44,000 miles from pole to pole each year, flying between its breeding grounds in Greenland in the north and the Weddell Sea on the shores of Antarctica in the far south, in a lifetime spent in perpetual summer.

Small tracking devices attached to the terns' legs have allowed their movements to be monitored in more detail than ever before. Scientists from the British Antarctic Survey found that the birds do not immediately head south from Greenland, but first spend almost a month at sea, in the middle of the North Atlantic, before continuing down the coast of north-west Africa.

Around the Cape Verde Islands, the researchers were surprised to find that about half the birds carried on south along the African coast while the rest crossed the Atlantic to follow a parallel route down the east coast of South America.

All the terns studied escaped the northern winter by flying to Antarctic waters, where it is summer at that time of year. On the return trip, they again did not take the shortest route, but traced a giant 'S' shape.

These diversions took advantage of prevailing global wind systems to help the birds preserve energy, according to Carsten Egevang, from the Greenland Institute of Natural Resources.

"They paused in their southward migration to spend time in highly productive waters in the middle of the Atlantic Ocean," he said. "Clearly, Arctic terns have learned to 'fuel up' before crossing areas of ocean with limited foraging options."

Arctic terns feed from the water while on the wing and can live up to 34 years. When the scientists added up the total distance each bird flew during its lifetime they found it equalled three round trips to the Moon – or more than 1.25 million miles.

The findings are published today in the journal Proceedings of the National Academy of Sciences.