[TN-Bird] Major discover in how birds migrate (VERY LONG)
- From: "Wallace Coffey" <jwcoffey@xxxxxxxxxx>
- To: "TN-birds" <tn-bird@xxxxxxxxxxxxx>
- Date: Tue, 28 Sep 2004 22:43:16 -0400
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The Homing Instinct
How Do Birds Navigate Fall Migrations? Scientists Say They've =
Found the Secrets
By Amanda Onion
Sept. 27, 2004 - Last Sunday Jack Meyer spotted 10 different =
warblers, a bald eagle and a couple varieties of hawks as the birds =
paused in New York City's Central Park during their epic journeys to =
winter feeding grounds.=20
It was a good day for Meyer, a retiree who earns a little money on =
the side by guiding birding tours and identifying the traveling birds. =
But when it comes to understanding how the birds manage to navigate =
their twice-a-year journeys to summer and winter feeding grounds, the =
veteran admits he's confounded.=20
"I understand a little about it, but not much," he said.=20
He's not alone. The question of how migrating birds manage to fly =
for hundreds, sometimes thousands of miles, often at night, to end up in =
the same spot year after year has been nagging biologists, even =
physicists, for decades. Do they hone in on landmarks or familiar =
smells? Do they orient themselves using the sun or Earth's magnetic =
field?=20
Finally, after years of research, it appears scientists have found =
the answer: Yes.=20
It turns out that birds are sophisticated navigators who use a mix =
of methods to keep their flights on course.=20
Seasoned Navigators=20
Research has shown they use landmarks as they make their way =
overland and homing pigeons (which are not migrating birds) even follow =
highways and make their turns at marked exits. New work has revealed =
that migrating birds also rely on proteins in their eyes that act like =
compasses to keep themselves pointed north or south - depending on the =
season. And, because magnetic north shifts and can be an unreliable =
guide, they check the setting sun each evening to reset their course.=20
"People like myself have been studying this for a very long time," =
said Henrik Mouritsen of the University of Oldenburg in Germany. "How =
they get their magnetic sense is the last question that was left to be =
understood. Now we're pretty close."=20
To understand the birds' tricks, Mouritsen and his colleagues =
first tried to confuse some migrants.=20
Mouritsen and Martin Wikelski of Princeton University in New =
Jersey and William Cochran of the Illinois Natural History Survey in =
Champaign captured songbirds in Illinois and Iowa as they made their =
journeys north in the spring. Then they subjected the birds to a false =
magnetic field at dusk and released them.=20
Birds that had been exposed to the warped magnetic field took off =
in the wrong direction, but by the second night they had managed to =
correct themselves and point north once more.=20
This suggested that the birds use the setting sun to correct their =
internal compasses. It's a skill the birds need even if scientists don't =
grab them and throw off their magnetic compass because Earth's magnetic =
field shifts so magnetic north is often inconsistent with geographical =
north.=20
"It's such a simple and elegant mechanism that I would say it is =
widespread [among migrating birds]," said Wikelski, who published the =
results with his colleagues in an April issue of Science.=20
Which Exit for Florida?=20
For birds that have already flown a route before, plenty of =
evidence has suggested the travelers keep tabs on landmarks below to =
find their way. In the United States, geographical features tend to =
funnel the traveling birds into four main routes - the Atlantic, =
Mississippi, Central and Pacific flyways.=20
Migrating birds that have made the trip before become familiar =
with particular landmarks and smells along the way and use them to keep =
them on course. Some don't limit their visual cues to natural features.=20
A research group led by Hans-Peter Lipp of the University of =
Zurich in Switzerland recently used Velcro and glue to attach =
lightweight GPS logging devices on the backs of homing pigeons. The =
loggers provided data on the flight paths of pigeons as the birds winged =
for up to 75 miles in journeys home from various points around Rome.
After researchers mapped more than 200 flight paths over three =
years, it was clear the birds preferred to travel above highways. In =
some cases, the roads veered too far off course from the birds' routes. =
When this happened, some birds even waited to see an exit on the highway =
before making their turn.=20
"It's like the way a pilot of a small airplane may prefer to =
follow a road or powerline coincident with the compass rather than =
watching only the compass in the cockpit," explained Lipp.=20
Studies may have shown that birds use cues to find their way, but =
scientists have had trouble explaining the key mechanism behind their =
navigating feat - their internal compass. In the end, it took both =
physicists and biologists to figure out how the birds detect Earth's =
magnetic field.=20
Physicists were the first to suggest that migrating birds carry a =
protein in their eyes that are activated by blue and green light. When =
activated by this light (which is available both night and day), =
molecules in the proteins enter an active state and become sensitive to =
Earth's magnetic field.=20
It was a good theory, but it hadn't been tested in actual birds' =
eyes - until now.=20
Special Vision=20
Mouritsen dissected some garden warblers and located the proteins =
in the retinas of birds that had just been exposed to the blue green =
light and to a magnetic field. He found the proteins had been altered =
according to the change in the magnetic field and had sent this =
information to the birds' brains. The result is the birds see Earth's =
magnetic field superimposed over their normal vision like a pilot =
peering through a mounted target system.=20
"Imagine a light spot with concentric darker rings around it that =
moves around in the retina depending on where you look," said Mouritsen, =
who published the details of his research in Proceedings of the National =
Academy of Sciences. "This is what the bird sees - it's a weak pattern =
superimposed on what they see in the world."=20
There are still some questions Mouritsen and others still hope to =
answer, such as how the special protein sends information to the brain =
and whether any other sensing device might be at play. But, at least for =
now, they're satisfied to know even a little about how the birds manage =
their amazing feat.=20
Meyer, for his part, is more interested in the birds' moods as =
they wing their way north or south. He has found that birds heading =
north are usually in a hurry to start breeding and don't stay long at =
their rest stops. But those heading south for the summer tend to take a =
more leisurely pace.=20
This is why he prefers birding in the fall season.=20
"For the birder and birds, it's less hectic," he said. "You're =
less likely to miss some birds before they get back on the flyway."=20
--------------------------------------------
Forward by:
Wallace Coffey
Bristol, TN
=20
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