Molly Cavanaugh, age 13, of Anthon, Iowa, for her question:
What structure enables a bird to fly?
Fishes have fins, land animals have legs and the birds of the air have feathery wings. All of them get from here to there by using parts of their bodies to push against something. Legs move a land animal forward by pushing against the solid ground. Fishes use their fins to push and paddle through the water. The birds use their wings to push against the filmy air.
The ostrich and a few other birds are too bulky to get off the ground. Besides, their wings are too small and weak to support them in the air. But most birds are built for flight and almost every part of their bodies is structured to make this possible.
The average flying bird has a streamlined silhouette, with no protruding ears and such to obstruct the smooth flow of air currents around his body. He is a lightweight and everything about him weighs as little as possible. Flight, of course, requires a minimum of excess baggage. His slim, light bones have hollow pockets and there are other air sacs in various parts of his body. His clothing, naturally, is as light as feathers.
So much for the basic structure that a bird needs before he can try for his pilot's license. But even more important are his specially adapted front limbs known as wings. Their bones and, joints must be able to bend and stretch time after time without growing weary. The skin that covers his wings must be strong and stretchable. The feathers must be strong and durable pinions and a bird must be able to spread and close them like fingers. He also needs a bendable tail of stiff, spread able pinions.
With all this basic equipment, the average bird learns to fly without much instruction. Usually he sees his parents in flight and it seems quite natural to spread his wings and try to do likewise.
Chances are, he soon learns that there are a few more skills to this aerial feat. The main trick is to learn the upbeats and downbeats and how to co ordinate this motion with his tail and the rest of his body.
To us, the air seems like a filmy nothing and we learn early in life that we can't depend on it to support our bodies. A bird refuses to accept this known fact and proceeds to disprove it. With each downbeat, he piles up cushions of crowded air under his wings and body. He pushes himself forward against these cushions of air, as he brings his wings up for the next downbeat.
There is more, much more to the skills of flying. But every part of a bird is structured for flight, which helps him to learn by doing what comes naturally. He spreads and closes his pinions to scoop up cushions of air and let it flow through his feathery fingers. He tips and spreads his tail to bank and turn, take off and land. And many kinds of birds learn to select rising currents of air to hold their light bodies aloft and let them soar without even beating their wings.