"Over the course of its life, it's flying to the moon three times and back"

Of all migrating birds, the Arctic tern flies the farthest -- braving cold, wind, storms, predators and starvation to travel from as far as upper Greenland to the shores of Antarctica.

But little has been known about how these birds, weighing less than 125 grams, make their grueling journey.

Now, for the first time, scientists using tiny geolocating devices have tracked the terns' migration, and discovered some surprising details.

The tern can fly an average of about 44,000 miles, nearly twice the distance that scientists had predicted -- and some individuals can fly more than 50,000 miles in a year.

"Over the course of its life, it's flying to the moon three times and back," said study coauthor Iain J. Stenhouse, an ornithologist who worked for the National Audubon Society for five years.

Scientists had guessed at the terns' routes, but had not been able to definitively test those hypotheses before now.

"We see them when they're breeding and think, 'Oh, they're nesting,' but the rest of the year we have no idea where these birds are," said Bridget J. Stutchbury, a professor in York University, Toronto, who runs a behavioral and conservation ecology lab. Stutchbury was not involved in the study.

Stenhouse and lead author Carsten Egevang of the Greenland Institute of Natural Resources found that the southbound trek is a relatively straight shot down the Atlantic. But the way north is much longer, curving in an "S" shape and potentially adding more than 1,000 miles to the journey.

Yet, the northbound birds were making the trip home much faster.

Flying north, the scientists realized, the birds were using the same trade winds merchants used centuries ago to speed their journey.

"They're really cranking," Stenhouse said.

Flying south, however, the terns made a weeks-long pit stop right after starting out -- delaying the journey by taking time to fuel up in the food-rich North Atlantic waters. Another surprising finding was that as they passed the Cape Verde Islands, birds from the same colony split up and parted ways. One group hugged the western coast of Africa. The other followed the eastern edge of South America. The two groups joined up again in Antarctica.

The study, published online this week in the journal Proceedings of the National Academy of Sciences, was made possible by new technology.