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currents can allow a hawk to cover hundreds of miles in a single day while using a min-
imum of energy.
A hawk may cover considerable ground seeking out thermals and updrafts, although
experienced birds learn to search near shorelines and above pavement for a thermal,
where the temperature is at least a few degrees warmer than nearby water or vegetated
ground, causing the warmer air to rise, or to search near bluffs and tall buildings for an
updraft, because such obstructions force oncoming winds upward. Air currents are invis-
ible, but hawks can easily see exactly where a thermal or updraft is by watching for other
hawksspiraling upward.Theyaredrawnnottotheotherhawksbuttotherisingair.Over
time on a sunny morning, hundreds or thousands of hawks can be seen spiraling on the
same thermals.
SEE ALSO: pages 71 and 218 for more on hawk migration.
Q Why do geese fly in a V?
A TherearetwoadvantagestoaVformation.Thefirstisaerodynamic.Asasinglegoose
flaps through the air, it disturbs the air by creating wingtip vortices. These vortices are
generally undesirable because they create a downwash that increases drag on that wing.
However, this downwash is also accompanied by an upwash that can help a bird flying
behind and slightly above it, providing lift and reducing drag. This means that the second
bird doesn't have to flap as hard or often to maintain the same speed as the first.
Although the birds trailing the first bird benefit most, the first bird also saves energy
by having birds flying behind it, because their presence helps dissipate the upwash. In a
long V, the birds saving the least energy are the one in front and the two pulling up the
rear. That's why birds flying in formation shift positions fairly often. In one study, re-
searchers monitored pelican heartbeats as the birds flew. They found that the heart rates
of pelicans flying in formation were much lower than those of pelicans flying alone.
The second reason birds fly in a V is for the same reason that Air Force jets do — to
more easily maintain visual contact with one another.
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