Graphics Reference
In-Depth Information
FIG 4.60
The upward and
downward actions of the body of a
large bird during flight.
but the feathers slide across one another to maintain a perfectly smooth
and streamlined wing surface that keeps drag to a minimum. The action of
flapping is often rather like a human rowing a boat: The wings are stretched
forward in preparation for the downward motion, and as the wing moves
upward it is half folded to reduce surface area and resistance.
The cost of flying in terms of energy consumed is very high; a bird's
streamlining is very effective in keeping this energy consumption to a
minimum. Flying requires more energy than walking, yet overall there is a
lower cost for flying than in ground-based travel. If a small mammal and a bird
of a comparable size were to travel for the same amount of time, the mammal
would use a great deal less energy than the bird—around three times less.
However, the mammal would travel around 10 times more slowly than the
bird. If each were to be limited in the consumption of energy, the bird would
cover around four times the distance as the mammal for the same amount of
energy. Flying does cost a lot of energy, but it requires less total energy than
walking over distances.
The speed of powered flight varies a great deal from species to species. A
peregrine falcon can fly at 200 miles an hour; it sweeps downward on its
preferred natural prey of pigeon, using gravity to assist in its speed, folding
back its wings to achieve a more aerodynamic shape and then powering
forward with powerful wing beats. Owls, on the other hand, hunt very slowly.
The shape of an owl's wing is broad and rounded to increase lift, but the
feathers have developed in such a way as to reduce noise.
Gliding
Gliding is perhaps the simplest form of flight and probably the earliest form
of flight in birds. Most mammals that use flight as part of their locomotion
depend on gliding. Indeed, the only mammals to have developed a mode
of powered flight are bats. Gliding is a simple process of ensuring that the
upward forces are great enough to oppose the downward force of weight,
