Bonnie Wallace, age 14, of Oliver, B.C., Canada, for her question:

IS IT TRUE THAT BIRDS HAVE HOLLOW BONES?

There are some 9,000 different birds in the world, and most of them can fly. True, some birds cannot fly any better than you can. Such flightless birds as the ostrich, for example, get from here to there on strong, powerful legs. And the penguin is blessed with marvelous flippers for rapid swimming prowess. 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. As sleek as an airplane, he has no protruding parts such as ears to obstruct the smooth flow of air currents around his body. To get into the air and stay there, he has thrown overboard all excess baggage not concerned with flight. And perhaps his most successful weight reduction program can be found in his skeleton.

A bird's skeleton is much lighter than a mammal's of comparable size. This is surprising when you stop to consider that a bird's bones must be super strong to withstand the sudden stresses of its daily aerial acrobatics. Looping the loop, sudden turns and power dives could snap weaker bones. But the birds have a secret    their hollow bones are not only lighter, they are also built in such a way that makes them very strong.

Many of the long, round bones in a bird's skeleton are indeed hollow. Not as hollow as the inside of a cardboard tube, but still hollow. Here and there, inside the bones, are special braces that reinforce them like the struts found inside airplane wings. A bird's skull is especially light because of the network of bony braces that support it, and there are no weighty teeth to worry about.

In addition to lighter bones, a bird's skeleton contains fewer bones than a mammal's, and many of their bones are fused together. This helps give a bird a more rigid framework that is necessary for flight. Their ribs are long, thin and flat. They allow plenty of movement for breathing and flying, yet they are light and strong. What's more, each rib overlaps its neighbor, reinforcing its own strength.   

Of course, birds have other features that enable them to soar throught the sky. Wings, feathers, air sacs in its body and naturally very powerful chest muscles all contribute. And to provide the fuel for its mighty muscles, birds concentrate on "high octane" foods, such as seeds, fruits, insects, nectar and the like.

A bird on the wing is a beautiful sight to behold. Once aloft, some soar for endless minutes without so much as a flap of their powerful wings. Banking and turning, riding the invisible air currents, they seem like magical, whisper soft shawdows in the sky. Little wonder that people around the world associate birds with freedom, strength and grace.