Migrating Thousands of Miles With Nary a Stop

[Cees Binkhorst]

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Groet / Cees

May 24, 2010
Migrating Thousands of Miles With Nary a Stop
http://www.nytimes.com/2010/05/25/science/25migrate.html
By CARL ZIMMER

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.

“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 crab eggs, which birds like the red
knot travel thousands of miles to eat. 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.”

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