BIRDS flying south for the winter cannot navigate by the stars alone,
according to new evidence from Germany. The night sky provides only rough
directional cues, whereas information from the Earth鈥檚 magnetic field is
essential if birds are to fly off on exactly the right heading.
Biologists know that birds have two complex navigation systems: one that
relies on the position of the stars and another that uses the Earth鈥檚 magnetic
field. Either one, researchers thought, was adequate to guide migrating birds on
their journeys.
But now researchers led by Wolfgang Wiltschko, a zoologist at Goethe
University in Frankfurt, have shown that, for garden warblers at least, stars
alone are not enough. 鈥淭he celestial rotation provides only a north-south axis,鈥�
says Wiltschko. 鈥淏ut the deviation from that axis is coded with respect to
magnetic information.鈥�
Advertisement
At the end of each summer, central Europe鈥檚 garden warblers set off towards
the southwest. This takes them to the Iberian peninsula, from where they head
south to Sierra Leone. From there they fly southeast towards South Africa. But
although they are born with these instructions, the birds need an external
reference system to get the flight path right.
The researchers raised two groups of warbler chicks until they were about six
weeks of age. Both groups were exposed to an artificial sky with 16 fake stars
rotating once per day to mimic the motion of real stars. But while one group
experienced the Earth鈥檚 magnetic field, it was cancelled out with magnetic coils
for the other group.
In mid-August, at the onset of the birds鈥� migratory restlessness, Wiltschko
and his colleagues recorded the birds鈥� activity to determine the direction in
which they intended to fly. In this week鈥檚 issue of Nature (vol 383, p
158) they report that warblers that had been exposed to the stars and the
Earth鈥檚 magnetic field oriented themselves in the correct southwesterly
direction. But the other birds prepared to set out almost due south. 鈥淭hey would
have had to cross the Alps and the central Sahara,鈥� says Roswitha Wiltschko,
another member of the team.鈥滻t would be much harder for them.鈥�