Page 54 - The Miracle of Migration in Animals

THE MIRACLE OF MIGRATION IN ANIMALS

Advantages of High-Altitude Flight

Some birds migrate at seemingly impossible altitudes. For in-
stance, dunlin, knot and certain other small migrating birds fly at a
level of 7,000 m (23,000 feet), the same altitude used by aircraft.
Whooper swans have been seen flying at 8,200 m (27,000 feet). Some
birds even reach the stratosphere, the layer of thin atmosphere, at an
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altitude of between 8 and 40 kilometers (5 and 25 miles). Bar-headed
geese cross the Himalayas at an altitude of 9,000 meters (29,529 feet),
close to where the stratosphere begins. 12
It is not known exactly how birds determine the altitude at
which they fly, but high flying does give a number of advantages. It
may let them locate familiar landmarks, fly over fog or clouds, and
surmount physical barriers like mountain ranges. At very high alti-
tudes, the air is cooler and this means reducing water loss for the
birds. 13
Although birds gain advantages from flying high, there could
be certain disadvantages. For example, concentrations of oxygen at
this altitude are less than a third of what they are at sea level.
However, birds do experience no difficulty because their systems are
created to cope with breathing at high altitudes. Geese and other
birds have very efficient forms of oxygen-carrying molecule hemo-
globin in their blood to deal with this low level of oxygen and in ad-
dition, there is a high density of capillaries to transport this oxygen to
their flight muscles. The “avian lung” structure unique to birds
moves the air in their lungs in a single direction, meaning that the
bird constantly breathes in clean air and thus can use oxygen in the
atmosphere in the most efficient way.
How migrating birds manage to tolerate the cold is still un-
known. At high altitudes, the temperature can fall below -50°C

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