Birds fly best on a full tank
Food and formation help birds fly efficiently.
18 October 2001
Swimming after a heavy meal may not be wise - but flying is
another matter. Birds fly more efficiently when loaded with
food, new research suggests, helping to explain how they
can migrate thousands of kilometres without stopping1.
And a second study has confirmed the century-old
suspicion that birds fly in a V formation to save substantial
amounts of energy2.
Anders Kvist at Lund University in Sweden and his colleagues
looked at flying efficiency in red knots, small waders that
double in size for their annual migration from Siberia to Africa. Fully
fed, red knots flying in a wind tunnel for 6-10 hours extracted
significantly more power from each unit of food.
This might help to explain why birds often make long non-stop flights
even when they don't have to cross an ocean or desert, says Kvist.
"Since efficiency increases when the birds are heavy, it might not be as
bad to make long flights as people thought."
The research flies in the face of computer predictions that birds are
less efficient when full. Says bird aerodynamics specialist Jeremy
Rayner of the University of Leeds: "It's a major advance, because it has
disproved something we've held on to for a long time."
The finding is "extremely unexpected", agrees John Speakman
who works on animal energy use at the University of Aberdeen. "This
changes our whole view of migrational strategies in terms of how
much fat birds should deposit to cross, say, the Sahara Desert."
Understanding the relationship between food and flight might help
ecologists to measure the impact of habitat change on migratory birds,
Speakman says. "If you're deciding whether to flood an estuary, for
example, this could help you make more sensible predictions about how
it will affect birds that use the estuary as a stopover."
It is unclear how birds increase their efficiency when migrating, Kvist
says. Puzzlingly, they don't adopt the most economical strategy at all
Kvist speculates that when birds are breeding they may keep reserves of
strength for sudden manoeuvres such as speeding up or swerving to avoid
Birds also conserve fuel by flying in V formations. By
measuring heart rates, researchers in France now
have proof that pelicans use 11-14% less energy flying
together, even when they are not perfectly positioned
to take advantage of the wake from those in front of
Configured flight may create a stream of air that allows
birds to glide longer, suggests Henri Weimerskirch, the biologist at
the National Centre of Scientific Research at Villiers en Bois, who led
the study. "If you look closely, you see that the birds at the back are
gliding more than the leader."
People have been asking whether V formations are more efficient for
more than 100 years, Speakman says, but no one had measured energy
savings before. "They took a century-old problem and went to the heart
of it," he says.
1.Kvist, A., Lindstrom, A., Green, M., Piersma, T. & Visser, G. H.
Carrying large fuel loads during sustained bird flight is cheaper than
expected. Nature, 413, 730 - 732, (2001).
2.Weimerskirch, H., Martin, J., Clerquin, Y., Alexandre, P. &
Jiraskova, S.. Energy saving in flight formation. Nature, 413, 697 -
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