http://news.bbc.co.uk/2/hi/science/nature/3629775.stm
Friday, 16 April, 2004, 00:08 GMT 01:08 UK
Migrating birds rely on sunsets
By Tim Hirsch
BBC environment correspondent
US scientists believe they have made an important breakthrough in the
mystery of how migrating birds manage to navigate thousands of
kilometres and arrive at exactly the same spot each year.
Laboratory experiments in the past have suggested the birds may use a
number of cues, including sunlight, stars and the Earth's magnetic
field, which they can detect inside their bodies.
But this new study, reported in the journal Science, involved tracking
a group of thrushes across hundreds of miles in the American Midwest
and finding out what happened when they were deliberately confused by a
man-made magnet and knocked off course.
The birds were captured during their northward migration and released
at nightfall when they would normally continue their journey - having
been fitted with tiny radio transmitters.
Remote chase
But just before setting off, the birds were exposed to the artificial
magnetic field pointing east.
Their flight path was then tracked by researchers chasing animals for
six days in a 1982 Oldsmobile with an antenna mounted on top.
On some nights, the scientists were held up by traffic police who were
suspicious of the battered car zooming along remote roads laden with
electronic equipment.
But the experiment yielded intriguing results. On the first night, the
songbirds headed west, apparently knocked off course by the magnet.
On subsequent nights, they managed to correct this mistake and
continued northwards towards their destination.
The researchers concluded that each night the thrushes must have
"recalibrated" their inbuilt compass from the position of the setting
Sun.
Reliable navigation
This helps to explain two enigmas surrounding the "flying by compass"
theory.
First, the position of the magnetic North Pole shifts significantly
each year, so magnetic signals alone would not be reliable, especially
in higher latitudes.
Also, bird compasses cannot distinguish between north and south, so you
would expect them to be confused when crossing the equator.
Amazingly then, this study does seem to confirm that birds can "reset"
their navigation systems daily by comparing the direction of the sunset
with the magnetic signals they detect.
The research was conducted by William Cochran, at the Illinois Natural
History Survey in Champaign, and colleagues.
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