Tens of thousands of bar-tailed godwits are taking advantage of favorable winds this month and next for their annual migration from the mud flats and muskeg of southern Alaska, south across the vast expanse of the Pacific Ocean, to the beaches of New Zealand and eastern Australia.
They are making their journey of more than 7,000 miles by flapping night and day, without stopping to eat, drink or rest.
“The more I learn, the more amazing I find them,” said Theunis Piersma, a professor of global flyway ecology at the University of Groningen in the Netherlands and an expert in the endurance physiology of migratory birds. “They are a total evolutionary success.”
The godwit’s epic flight — the longest nonstop migration of a land bird in the world — lasts from eight to 10 days and nights through pounding rain, high winds and other perils. It is so extreme, and so far beyond what researchers knew about long-distance bird migration, that it has required new investigations.
In a recent paper, a group of researchers said the arduous journeys challenge “underlying assumptions of bird physiology, orientation, and behavior,” and listed 11 questions posed by such migrations. Dr. Piersma called the pursuit of answers to these questions “the new ornithology.”
The extraordinary nature of what bar-tailed and other migrating birds accomplish has been revealed in the last 15 years or so with improvements to tracking technology, which has given researchers the ability to follow individual birds in real time and in a detailed way along the full length of their journey.
“You know where a bird is almost to the meter, you know how high it is, you know what it’s doing, you know its wing-beat frequency,” Dr. Piersma said. “It’s opened a whole new world.”
(Sea-faring Polynesian cultures, the scientists wrote in the paper, knew about the migrations long ago and used the birds to assist in navigation.)
The known distance record for a godwit migration is 13,000 kilometers, or nearly 8,080 miles. It was set last year by an adult male bar-tailed godwit with a tag code of 4BBRW that encountered inclement weather on his way to New Zealand and veered off course to a more distant landing in Australia. He had flapped his wings for 237 hours without stopping when he touched down. (In the last week, he has left Alaska again and is en route to his southern destination.)
The Fascinating World of Birds
Other birds do stay aloft for long periods using a technique called “dynamic soaring,” while godwits power themselves by continuous flapping, which takes far more energy.
The globe-trotting birds are in search of an endless summer, and some 90,000 or so depart Alaska from the Yukon-Kuskokwim Delta and environs, where they breed and raise their young. Both Alaska and New Zealand are rich in foods that godwits like, especially the insects in Alaska for newly hatched chicks. And New Zealand has no predatory falcons, while Alaska offers secure habitat.
Once they reach New Zealand and the austral summer, the sleek birds — with mottled brown-and-white aerodynamic wings; cinnamon-colored breasts; long, slender beaks; and stilt-like legs — feed on glistening mud flats until March, when they begin their journey back north.
“I tell people try exercising for nine straight days — not stopping, not eating, not drinking — to convey what’s going on here,” said Robert E. Gill Jr., a biologist with the U.S. Geological Survey in Anchorage who has studied the birds in Alaska since 1976. “It stretches the imagination.”
Distances vary, but all told, in a year, the godwits cover some 30,000 kilometers, or nearly 18,720 miles, because they take a less direct route to return north in March. They fly nonstop from New Zealand to China’s Yellow Sea and its rich tidal flats, where they refuel, and then return to Alaska. And they are proficient at the incredibly risky endeavor; the survival rate is more than 90 percent.
“It’s not really like a marathon,” said Christopher Guglielmo, an animal physiologist at Western University in London, Ontario, who studies avian endurance physiology. “It’s more like a trip to the moon.”
The journey of these ultra-endurance athletes is made possible by a suite of adaptations.
Godwits are avian shape-shifters, endowed with an unusual plasticity. Their internal organs undergo a “strategic restructuring” before departure. The gizzards, kidneys, livers and guts shrink to lighten the load for the trans-Pacific journey. Pectoral muscles grow before takeoff to support the constant flapping the trip requires.
They are built for speed, with aerodynamic wings and a missile-shaped body. The only baggage the birds carry is fat, by gobbling up insects, worms and mollusks to double their weight from one to two pounds before embarking on their trip. Because godwits directly use fat to fuel their flight, Dr. Guglielmo in one paper called them “obese super athletes.”
Bird lungs are the most efficient lungs of any vertebrate and help the godwits’ performance in the thin atmosphere of higher altitudes. Bar-tailed godwits in Russia have recently been documented flying at altitudes of three to four miles above ground.
No other birds make the same length of powered migration under such punishing conditions, but recent research shows that common swifts stay airborne for virtually all of the 10 months when they’re not breeding or nesting, although they eat and drink during that time.
Climate change and other factors are bearing down on migratory shorebirds around the world. In Alaska, for example, rising sea levels are lapping at the nesting sites of godwits and important grassy habitat is being ‘shrub-ified’ — taken over by shrubs — because of warmer temperatures. Experts are also worried about avian influenza, which has spread globally among wild birds this year and is often lethal.
Wayfinding among the godwits is among the biggest questions recent studies have prompted. “What mechanisms explain birds acting as if they possess a Global Positioning System?” researchers asked. Crossing a nearly featureless Pacific Ocean without navigational cues required an internal “map to define position and a compass to tell direction,” they said. The birds find their way back to the same specific sites at the end of their flight, something they do for each of the 15 or 20 years of their lives.
“They have figured out the aerosphere they live in,” Dr. Gill said. “They can predict when to leave and when not to leave, how high to fly, and they know exactly where they are and they know their destination.”
The godwits probably rely on several cues for navigation, especially the sun and stars. Some experts believe that they may be able to sense magnetic lines on the planet through a process called quantum entanglement.
The birds also possess an uncanny knack for weather forecasting.
“They know what conditions to leave on that will not only provide wind at the start that is favorable, but throughout their entire flight,” Dr. Gill said. “They can piece the puzzle together in terms of what the conditions are in Alaska and between there and Hawaii, between Hawaii and Fiji, and between Fiji and New Zealand. How migration abilities are passed on to the next generation — whether genetically or learned or a combination — is still unknown.
“You study adults, and you think these birds just have it down, they are super robots, they are amazing,” said Jesse Conklin, an independent researcher at the University of Groningen who studies the species. “But when you study young birds, they make mistakes and do all kinds of weird stuff. So they weren’t just born with this routine.”
Incredibly, it is possible that three-month-old godwit juveniles fly their nonstop maiden voyage without adult supervision. That has yet to be confirmed.
The energetics of their nonstop migration are also a conundrum. Current models say the birds should conk out after three or four days, yet they fly for more than a week. “We can’t explain the physiology that allows them to do this,” Dr. Guglielmo said. “We know what the energy costs should be from wind tunnel experiments, but when we try to use our models, the energy costs we know they used are much lower.” The birds use half or less of the energy expected.
One answer may be that the birds can lower their metabolic rate on these journeys, burning far less energy than they would for other kinds of flying. “Are they going into a suspended animation state when they are doing these monster flights?” Dr. Guglielmo asked. “I don’t think they are in a normal physiological state when they are doing this,” he said, adding they might enter into a state like something akin to “marathon runners getting into the zone.”
Whether or how the birds sleep is another mystery. It’s been shown that some bird species are capable of unihemispheric sleep, that is, putting one half of their brain to bed while using the other half to fly. Others believe the birds don’t sleep at all but catch up on their rest when they reach New Zealand.
Experts believe the birds communicate frequently, especially about the timing and safety of their trip. Some suggest that the birds gather to create a kind of group mind that helps them make decisions on important matters and take votes on migration, among other things.
“It’ll be near hurricane weather and a bird will be stamping around the estuary, calling, trying to get someone to go with her,” Dr. Conklin said. “I watched a bird do this for five days straight. Her clock said go, and everybody else said no. She got outvoted.”
She stayed, he said, “but as soon as the weather turned, she was in the first flock out.”