The Arctic tern’s global trek

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.

More traditional tracking devices are equipped to regularly transmit information via satellite -- which requires a lot of power and a heavy battery. These can only be used to track the migration patterns of larger birds, such as albatrosses.

Attaching a device of this size to a tern’s leg “would be like giving me a 100-pound pack to carry up a mountain,” Stutchbury said.

The new geolocators, however, are small and more energy efficient. Instead of sending out signals, the 1.4-gram devices simply keep track of light levels. This allows scientists to pinpoint the birds’ location on a given day.

It’s marvelously simple, Stenhouse said -- with one caveat. Because the older, satellite transmitters constantly relay information, a researcher never has to lay eyes on the birds that carry them again. Not so with the new geolocators. The scientists had tagged 50 birds in Greenland and 20 in Iceland. At the Greenland nesting ground, Stenhouse and Egevang had to recapture as many of the geolocator-toting terns as they could -- a somewhat troublesome task.

“We spent a month on a tiny little island with the birds and a few walruses,” Stenhouse said. They had to hunt through thousands of identical birds, squinting at their tiny legs. They eventually found 10 from Greenland and one from Iceland -- aided, it turns out, by the birds’ aggressive behavior.

When flying, the birds generally tucked their legs in, making the devices impossible to spot. The devices were visible, however, when the birds dive-bombed the two scientists’ foreheads.

“When I saw one, I’d call out to Carsten, and we’d follow it and try to keep track of it . . . that was the difficult part of the whole thing, really,” Stenhouse said.

Stenhouse said the findings raised interesting issues for scientists who study migration. For example: Was the divergence on the southbound trip born of experience, or instinct?

“We’re not sure yet whether that’s some sort of ancestral thing,” Stenhouse said. The study, he added, “probably opens up as many questions as it answers.”

amina.khan@latimes.com