By CARL ZIMMER
Published: May 24, 2010
In 1976, the biologist Robert E. Gill Jr. came to the southern coast of Alaska to survey the birds preparing for their migrations for the winter. One species in particular, wading birds called bar-tailed godwits, puzzled him deeply. They were too fat.
TRACKERS On Alaska’s Yukon River delta, scientists implant a transmitter in a bristle-thighed curlew. The birds fly as far as 6,000 miles without a stop.
“They looked like flying softballs,” said Mr. Gill.
At the time, scientists knew that bar-tailed godwits spend their winters in places like New Zealand and Australia. To get there, most researchers assumed, the birds took a series of flights down through Asia, stopping along the way to rest and eat. After all, they were land birds, not sea birds that could dive for food in the ocean. But in Alaska, Mr. Gill observed, the bar-tailed godwits were feasting on clams and worms as if they were not going to be able to eat for a very long time.
“I wondered, why is that bird putting on that much fat?” he said.
Mr. Gill wondered if the bar-tailed godwit actually stayed in the air for a much longer time than scientists believed. It was a difficult idea to test, because he could not actually follow the birds in flight. For 30 years he managed as best he could, building a network of bird-watchers who looked for migrating godwits over the Pacific Ocean. Finally, in 2006, technology caught up with Mr. Gill’s ideas. He and his colleagues were able to implant satellite transmitters in bar-tailed godwits and track their flight.
The transmitters sent their location to Mr. Gill’s computer, and he sometimes stayed up until 2 in the morning to see the latest signal appear on the Google Earth program running on his laptop. Just as he had suspected, the bar-tailed godwits headed out over the open ocean and flew south through the Pacific. They did not stop at islands along the way. Instead, they traveled up to 7,100 miles in nine days — the longest nonstop flight ever recorded. “I was speechless,” Mr. Gill said.
Since then, scientists have tracked a number of other migrating birds, and they are beginning now to publish their results. Those results make clear that the bar-tailed godwit is not alone. Other species of birds can fly several thousand miles nonstop on their migrations, and scientists anticipate that as they gather more data in the years to come, more birds will join these elite ranks.
“I think it’s going to be a number of examples,” said Anders Hedenström of Lund University in Sweden.
As more birds prove to be ultramarathoners, biologists are turning their attention to how they manage such spectacular feats of endurance. Consider what might be the ultimate test of human endurance in sports, the Tour de France: Every day, bicyclists pedal up and down mountains for hours. In the process, they raise their metabolism to about five times their resting rate.
The bar-tailed godwit, by contrast, elevates its metabolic rate between 8 and 10 times. And instead of ending each day with a big dinner and a good night’s rest, the birds fly through the night, slowly starving themselves as they travel 40 miles an hour.
“I’m in awe of the fact that birds like godwits can fly like this,” said Theunis Piersma, a biologist at the University of Groningen.
Not long ago, ornithologists had far lower expectations for birds. Ruby-throated hummingbirds, for example, were known to spend winters in Central America and head to the United States for the summer. But ornithologists believed that the hummingbirds burned so much fuel flapping their wings that they simply could not survive a nonstop trip across the Gulf of Mexico. They were thought to have flown over Mexico, making stops to refuel.
In fact, ruby-throated hummingbirds returning north in the spring will set out from the Yucatán Peninsula in the evening and arrive in the southern United States the next afternoon.
In the 1960s, zoologists began to track bears and other mammals with radio collars, and then later moved on to satellite transmitters. All the while, ornithologists could only look on in envy. The weight and drag of the trackers made them impossible to put on migrating birds.
Over the past decade, however, transmitters have finally shrunk to a size birds can handle. In Mr. Gill’s first successful experiment with bar-tailed godwits, he and his colleagues slipped a battery-powered model weighing just under an ounce into the abdominal cavity of the birds, which weigh about 12 ounces and have a wingspan of 30 inches.
The epic odyssey that those transmitters recorded spurred Mr. Gill and other researchers to gather more data, both on bar-tailed godwits and other species. And even as they planned their experiments, tracking technology got better. This summer, for example, Mr. Gill will implant bar-tailed godwits with transmitters that weigh only six-tenths of an ounce.
Still, most migrating birds are so small that even a transmitter of that weight — about the same as three nickels — would be an intolerable burden. Fortunately, researchers have been able to scale down a different kind of tracking device. Known as a geolocator, it can get as light as two grains of rice, less than two-hundreths of an ounce. “Now we can track really small birds,” Dr. Hedenström said.
Geolocators can get so small because they do not communicate with satellites. Instead, they just record changing light levels. If scientists can recapture birds carrying geolocators, they can retrieve the data from the devices and use sophisticated computer programs to figure out the location of the birds based on the rising and setting of the sun.
In 2007, Carsten Egevang of Aarhus University in Denmark and his colleagues attached geolocators to Arctic terns nesting in Greenland. Based on years of bird spotting, the scientists knew that the terns migrated to the Southern Ocean around Antarctica and then returned to the Arctic the following spring. But they did not know much more than that. “It was all based on snapshots,” Dr. Egevang said.
In 2008, the scientists managed to capture 10 Arctic terns that had come back to Greenland. It then took them months to make sense of the data. “You have to use three kinds of special software,” Dr. Egevang said. “It takes quite a long time.”
The researchers reported this February that the Arctic terns flew from Greenland to a region of the Atlantic off the coast of North Africa, where they spent about three weeks. Unlike bar-tailed godwits, which wade on beaches for food, Arctic terns are ocean birds that can dive for fish in the open sea.
The Arctic terns then resumed their journey south. They spent five months in the Southern Ocean. “They probably just stayed on an iceberg and fished,” Dr. Egevang said.
In the spring, the terns then returned to the Arctic, often hugging the coasts of South America or Africa along the way. All told, the birds logged as much as 49,700 miles on their geolocators, the longest migration ever recorded. Over the 30-year lifetime of a tern, it may migrate about 1.5 million miles — the distance a spaceship would cover if it went to the moon and back three times.
Other scientists are now placing geolocators on small wading birds as well. In a paper to be published in the Wader Study Group Bulletin, a team of ornithologists describe attaching geolocators to four ruddy turnstones. The birds left northern Australia in May 2009 and flew nonstop to Taiwan, a distance of 4,700 miles.
After a few days in Taiwan, the ruddy turnstones took flight again, making a series of trips northward until they reached Alaska. At the end of the summer, three of the four birds took the same route back south. The fourth struck out on a different path. It flew 3,800 miles nonstop to the Gilbert Islands in the Pacific. From there, it flew 3,100 miles back to Australia.
Mr. Gill and his colleagues have recorded similar odysseys from other wading birds, using satellite transmitters. They found that bristle-thighed curlews fly as far as 6,000 miles without a stop, traveling from Alaska to the Marshall Islands. They have also recorded whimbrels flying 5,000 miles nonstop from Alaska to Central America.
This spring, scientists are attaching geolocators to more birds, and they expect to find new champions. One population of red knots, for example, is now arriving in Delaware Bay from its wintering grounds 5,500 miles away in Argentina. “My bet is that a lot of them make it in one go,” Dr. Piersma said.
The long journeys these transmitters are revealing pose a biological puzzle. Dr. Piersma and other scientists are trying to figure out how the birds manage to push their bodies so far beyond most animals, and why.
As Mr. Gill observed when he first observed bar-tailed godwits, a long journey requires a lot of food. It turns out that long-distance migrators will enlarge their liver and intestines as they feed, so that they can convert their food as fast as possible. They build up large breast muscles and convert the rest of their food to fat.
By the time the birds are ready to leave, their bodies are 55 percent fat. In humans, anything more than 30 percent is considered obese. But as soon as the birds are done eating, their livers and intestines become dead weight. They then essentially “eat” their organs, which shrink 25 percent. The birds use the proteins to build up their muscles even more.
Once they take flight, the birds take whatever help they can get. Bar-tailed godwits time their departure with the onset of stormy weather, so that they can take advantage of tailwinds. “That gives them an extra push,” Dr. Hedenström said.
The birds then fly for thousands of miles. How they get to their final destinations remains a mystery. One thing is clear: they somehow know where they are, even when they are flying over vast expanses of featureless ocean. “It’s as if they have a GPS on board,” Dr. Piersma said.
A bird like a bar-tailed godwit cannot rely on the tricks used by birds that take short migrations. They cannot follow landmarks, for example. Some birds use the Earth’s magnetic field to navigate. But they do so by sensing the tilt of the field lines. At the equator, the lines run parallel to the surface, making them useless for birds that have to travel between hemispheres. Dr. Piersma suspects that when birds travel several thousand miles, they have to combine several different navigation tricks together.
As spectacular as these migrations may be, it may not take long for birds to evolve them. Long-distance migrators are closely related to short-distance birds. It is possible that many birds have the potential to push themselves to make these vast journeys, but they do not because the costs outweigh the benefits.
When animals raise their metabolism above four or five times their resting rate (the Tour de France level), they can become so exhausted that they become very vulnerable to predators. They can even become more prone to getting sick. Birds that go on long migrations may have escaped this tradeoff.
Birds like the bar-tailed godwit have found places like the coast of Alaska where the supply of food is high and predators are scarce. By flying over the open ocean, they continue to avoid predators. They may also reduce their odds of picking up a parasite from another bird.
Their destinations are also safe enough for them to recover. Bar-tailed godwits that arrive in New Zealand face no predators, and so they can simply rest. “They just look exhausted. They’ll land and just go to sleep for several hours before they do anything else,” Mr. Gill said.
Unfortunately, some of the habitats on which these endurance champions depend are under serious threat. In the Delaware Bay, for example, fisherman are scooping up horseshoe crabs to bait traps for eel and conch. Birds like the red knot, which travel thousands of miles, land on Delaware Bay beaches to feast on the eggs of the crabs. When bar-tailed godwits return to Alaska in the spring, they make one stop along the coast of China and Korea, a favorite spot for many other migrating birds. The coastal wetlands there are disappearing fast, and many migrant birds are in decline.
“I hope we have these birds to study 100 years from now,” Dr. Piersma said. “But sometimes I wonder.”
This article has been revised to reflect the following correction:
Correction: June 4, 2010
An article on May 25 about long-distance bird migration incorrectly described a threat posed to red knots in the Delaware Bay, where the birds land to feast on horseshoe crabs’ eggs and regain their strength. Fishermen collect the crabs themselves (for bait in eel and conch traps), not their eggs.