Everything about a bird is made for flight. Apart
from feathers and wings, the internal structure and internal power system of
birds make their flight possible. In order to fly, every part must give maximum
power with a minimum of weight. There is a weight limit for a flapping bird. The
heavier the animal, the bigger its wings need to be. The bigger the wings, the
more muscle is needed to move them.
The best way to fly is to be light, so most birds are small. Birds have
feathers that are very light. The bones that birds have are also made for
lightness. Birds have fewer bones than most animals. The bones they have are
hard but thin. Some birds have bones weighing less than their feathers. In birds
many bones are fused together. In this way not as many muscles are needed.
The structure of a bird's eyes is very special. Birds have 3 eyelids. The
actual image a bird sees depends on the placement of the eyes. The eyeballs of
birds are flatter than those of humans. And the retina of a bird's eye is 2
times as thick as a man's. A sparrow hawk can see 8 times more clearly than man.
Hearing is another sense that is strong in birds. The ears not only hear well
but are used for maintaining balance. This is critical during flight.
What's more, birds must have extra power to keep a constant body temperature.
Birds have a natural way to solve this power problem. All the inside organs of a
bird run at high speed. This makes for high energy, but shortens the life span
of the bird.
The kinds of foods eaten by birds must be "high-octane". These foods are high in calories.
They are seeds, nuts, fruits, fish, and rodents. At night a bird must rest and
their body functions slow down. A hummingbird would starve to death at night if
his heart, respiration, and body
temperature did not slow down.
To create energy from the foods they eat, birds must burn them up. This is
called combustion and oxygen is
needed to do this. Oxygen is taken into the body through the lungs where it
passes into the bloodstream. Birds need a very fast system to supply the energy
they need. Also, in birds there are air sacs next to the lungs. They never run
out of breath. The lungs of birds work faster than ours. Birds fly into breath.
The heart pumps all the blood through the system. A bird's heart beats much
faster than other animals. This makes the blood travel through a bird in
seconds. Because of this fast blood travel, a bird's body temperature is 7 or 8
degrees higher than a man's.
（北京外国语大学通讯员林静供稿 英语点津 Annabel 编辑）