HOW do birds meet the conflicting demands of migration? Their marathon
flights make feeding stops essential, yet every time they land they must avoid
predators. Now a study in Sweden has shown that for some migrating birds, the
threat of predators makes them guzzle food quickly and leave without putting on
as much weight as usual.
Thord Fransson of the Swedish Museum of Natural History and Stockholm
University and his colleague Thomas Weber were studying how the Swedish blackcap
warbler, Sylvia atricapilla, behaves during migration. The birds, which
travel to East Africa in winter, stop off along the way to feed for several days
at a time. 鈥淭he birds have no prior knowledge of the sites, their food value or
their predation risk,鈥� says Fransson.
To find out how the birds react to threats from predators at stop-off sites,
the researchers captured male blackcaps that were migrating for the first time.
They divided the birds into three groups, which were housed separately. For the
first group, they passed a fake predator鈥攁 stuffed sparrowhawk鈥攊n
front of the cage at randomly chosen times. A plastic bottle was flown in front
of others, and a third group had a peaceful, disturbance-free captivity.
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鈥淭he birds reacted to the sparrowhawk just as they do in the wild,鈥� says
Fransson. 鈥淭hey froze for 20 to 30 seconds and made very few contact calls.鈥�
Those that saw the flying bottles were more agitated than those left in peace,
but they did not freeze, confirming that the birds exposed to the stuffed hawk
probably thought it was real.
Fransson and Weber will report in Behavioural Ecology and
Sociobiology that the group that was exposed to the stuffed sparrowhawk ate
more food and put on weight almost twice as fast as the others. 鈥淲e think they
were trying to reach an appropriate level of fuel loading rapidly in order to
leave as soon as possible,鈥� says Fransson. He adds that although these birds put
on weight faster, they put on less weight than usual before trying to leave so
that they could fly more easily: 鈥淭his would give them a better chance of
avoiding aerial attacks.鈥�
鈥淭his is a very interesting study鈥�, comments Neil Metcalfe, a zoologist at
the University of Glasgow. 鈥淔eeding generally makes an animal conspicuous. So an
animal under greater risk of predation normally reduces its feeding rate.鈥� But
clearly, he says, the pressures of migration change the birds鈥� behaviour
patterns: 鈥淭hey are selecting an increased short-term risk to maximise long-term
谤别迟耻谤苍蝉.鈥�