像鸟儿那样自由自在的飞翔是所有人的梦想。然而，人最终也只能借着“铁翅膀”才能去领略天空的广阔，不能真正自由的飞翔。鸟儿为什么能够自由翱翔呢？看了下面的文章，您自然就会明白了。

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 eyelids¹. 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 hawk² 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 rodents³. At night a bird must rest and their body functions slow down. A hummingbird⁴ would starve to death at night if his heart, respiration⁵, and body temperature did not slow down.

To create energy from the foods they eat, birds must burn them up. This is called combustion⁶ 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.

high-octane: 高辛烷值的

respiration: 呼吸

combustion: 燃烧，氧化