Generally
birds follow the facetious advice often given to pilots --
"fly low and slow." Most cruise speeds are in the
20-to-30-mph range, with an eider duck having the fastest
accurately clocked air speed of about 47 mph. During a
chase, however, speeds increase; ducks, for example, can fly
60 mph or even faster, and it has been reported that a
Peregrine Falcon can stoop at speeds of 200 mph (100 mph may
be nearer the norm). Interestingly, there is little
relationship between the size of a bird and how fast it
flies. Both hummingbirds and geese can reach roughly the
same maximum speeds. There is, of course, a
considerable difference between the speed at which a bird
can fly and the speed at which it normally does fly. When
the bird is "around home" one might expect it to do one of
two things, minimize its energy use per unit time, that is,
minimize its metabolic rate, or m e the distance it travels
per unit of energy expended. A vulture loitering in the sky
in search of prey might, like the pilot of an observation
aircraft, maximize endurance; a seabird traveling to distant
foraging grounds might, like a Concorde encountering
headwinds on a transoceanic flight, maximize range. Staying
up longest does not necessarily mean going farthest. A bird
might be able to stay aloft 6 hours at 15 mph (maximum
endurance, covering 90 miles) or 5 hours at 20 mph (maximum
range, covering 100 miles). Birds can also choose to
maximize speed, as when being chased by a predator or racing
to defend a territory. Or they can choose some compromise
between speed and range. In order to determine what
birds normally do, Gary Schnell and Jenna Hellack of the
University of Oklahoma used Doppler radar, a device similar
to that used by police to catch speeders, to measure the
ground speeds of a dozen species of seabirds (gulls, terns,
and a skimmer) near their colony. They also measured wind
speeds with an anemometer, and used those measurements to
estimate the airspeeds of the birds. (The wind speeds were
generally measured closer to the ground than the birds were,
which led to some errors of estimation, since friction with
the surface slows air movements near the ground.) Airspeeds were found to be
mostly in the 10-to-40-mph range. The power requirements of
each bird at each speed could be calculated, and that
information was used to establish that the birds were
generally compromising between maximizing their range and
minimizing their metabolic rates with more emphasis on the
former. Airspeeds varied a great deal, but near the minimum
metabolic rate rather large changes in airspeed did not
require dramatic rises in energy consumption. For example, a
gull whose most efficient loiter airspeed was 22 mph could
fly at anything between 15 and 28 mph without increasing its
metabolic rate more than 15 percent. Most birds fly below 500
feet except during migration. There is no reason to expend
the energy to go higher -- and there may be dangers, such as
exposure to higher winds or to the sharp vision of hawks.
When migrating, however, birds often do climb to relatively
great heights, possibly to avoid dehydration in the warmer
air near the ground. Migrating birds in the Caribbean are
mostly observed around 10,000 feet, although some are found
half and some twice that high. Generally long-distance
migrants seem to start out at about 5,000 feet and then
progressively climb to around 20,000 feet. Just like jet
aircraft, the optimum cruise altitude of migrants increases
as their "fuel" is used up and their weight declines.
Vultures sometimes rise over 10,000 feet in order to scan
larger areas for food (and to watch the behavior of distant
vultures for clues to the location of a feast). Perhaps the
most impressive altitude record is that of a flock of
Whooper Swans which was seen on radar arriving over Northern
Ireland on migration and was visually identified by an
airline pilot at 29,000 feet. Birds can fly at altitudes
that would be impossible for bats, since bird lungs can
extract a larger fraction of oxygen from the air than can
mammal lungs. SEE: Wing
Shapes and Flight;
Soaring;
Flying
in Vee Formation;
Adaptations
for Flight. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.