Device tracks terns' migrations
A bird famous for long-distance travel makes the equivalent of three trips to the moon and back during its lifetime, researchers have found.
The Arctic tern makes a round trip of around 44,000 miles from pole to pole each year, flying between its breeding grounds in Greenland and the Weddell Sea on the shores of Antarctica.
Scientists including a team from the British Antarctic Survey (BAS) attached tiny tracking devices to terns to map their mammoth migrations.
They found that the birds do not immediately travel south from Greenland, but first spend almost a month at sea in the middle of the North Atlantic ocean.
They then continued 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 the same time of year. But they did not choose the shortest route back to Greenland. Instead, they traced a giant 'S' pattern northward through the Atlantic Ocean, making a detour of several thousand miles.
The diversion took advantage of prevailing global wind systems to help the birds preserve energy. Study leader Carsten Egevang, from the Greenland Institute of Natural Resources, said: "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.
"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."
Arctic terns 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. "This is a mind-boggling achievement for a bird of just over 100 grams," said Mr Egevang.
The findings are published in the journal Proceedings of the National Academy of Sciences.