7,000 Miles Non-Stop

FOR THE RECORD Researchers attached geolocators weighing as little as two grains of rice to Arctic terns like this one.

7,000 Miles Nonstop, and No Pretzels

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.
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.
............. 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.

snip

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.

snip

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.

snip

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.”

http://www.nytimes.com/2010/05/25/science/25migrate.html?sq=7000 miles&st=cse&scp=1&pagewanted=all

PB