Carla Wilburn, age 8, of Huntsville, Ala., for her question:
HOW DOES A BIRD FLY?
A number of birds, including the ostrich, penguin, emu, rhea and cassowary, are unable to fly. Scientists say that perhaps thousands of years ago their ancestors could fly, but through the years their wings became smaller and are now too small to support the birds' bodies in the air. There are about 9,000 species of birds throughout the world, and most can fly.
Birds can fly for exactly the same reason that airplanes can fly. Birds are able to fly mainly because the pressure of the air on top of their wings is less than the pressure below their wings.
When a bird's wings move forward, the air must travel farther and faster over the curved top surface of the wings than it does over the bottom surface. The pressure on top of the wing is less than the pressure below because of this difference in the speed of the air.
A bird in flight moves its wings downward rapidly. The wingtips trace a figure eight as they move through the air. The downward beat of the wings moves the bird forward.
A bird's wing feathers are arranged much like shingles on a roof. They change position, when the bird flies, however. On the downbeat of the wing, the feathers are pressed together so that no air can pass through them. The feathers open on the upstroke.
The large primary and secondary feathers help the bird most in flying. The tail feathers are used to steer and brake.
As a bird flies, it must have keen senses and perfect control over its muscles so it will not bump into things. Some birds, such as swallows, can fly at high speed directly to their nests through tangled tree branches. To fly so skillfully, a bird must be able to change direction almost instantly.
Kestrels and hummingbirds have a hovering ability that has been immitated by the helicopter. These birds beat their wings so rapidly that they can hover in the air without moving in any direction.
The heavier a bird is in relation to its wingspread, the faster it must beat its wings. Hummingbird wings beat from 60 to 70 times a second. The hummingbird is also the only bird that can fly backward.
Most birds use the art of flying as the way they can escape from their enemies.
Some ducks, about to take off and fly, use their feet to push the water as an assist as they skim along the surface of a lake or pond. In landing, the duck will spread its wings to slow a forceful drop. The wings act very much like a parachute.