Top Ten Scientific Facts : Evolution is False and Impossible., page 12

If bones aren't light, birds don't fly. it's that simple. Some bones are heavier and stronger than others, but ALL their bones are designed to be as light as possible.

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 biggest flying bird today is called the Great Bustard. They weigh as much as 32 pounds and are 4 feet long.

The best way to fly is to be light. So most birds are small. Birds have feathers that are very light weight. The bones that birds have are also made for lightness. Birds have less bones than most animals. The bones they have are hard but thin. The biggest bones in flying birds are the breast bone and the shoulder bones.

Some birds have bones weighing less than their feathers. The powerful flying birds have medium weight skeletons. Birds that don't fly have the heaviest skeleton. In birds many bones are fused together. In this way not as many muscles are needed. The biggest muscles a bird has are its flight muscles. These are very strong because they have to raise the entire body into the air.

sln.fi.edu...

Birds have a lot of adaptations for flight. Their bones are hollow and light, but strong, They have light feathers which catch the air. They can fold up their wings when they are not in use. Their lungs are extra efficient at extracting oxygen from the air (we huff and puff when running, and flying is much harder!). They eat huge amounts of high-energy food, relative to their body weight.

van.hep.uiuc.edu...

Many birds have hollow bones to reduce their weight. (Check this site for more information on the adaptations of the skeletons of birds.) These special bones are very strong though, to cope with the forces necessary for flight. (Birds that do not fly but dive need the weight of solid bones. Penguins are in this category.) Instead of relatively heavy teeth, birds have light jaws and beaks. The gizzard, present in many birds and used for digestion, is in the middle of the body, and helps keep the bird steady when flying.

Even the reproductive organs are specialized to reduce size. A female bird has only one ovary which is normally very small. During breeding season, this one ovary will increase up to 1,500 times in size. She will become more reliant on the male to bring her food during this time when her flying is impaired.

An interesting link is from the Cincinnati Zoo where they have an exhibit showing children their shape and weight if they were a bird! To achieve and maintain flight, a bird has to have tremendous muscles. Over 30 percent of a hummingbird's weight is in its flight muscles. In fact, the flapping motion is one of the most strenuous activities in the entire animal kingdom. The heavier the bird, the more difficult is flight. The top weight range of flying birds is about 20 - 30 pounds. Examples that come to mind of this upper limit are mute swans, California condors, and some pelicans.

birding.about.com...

If the wing bones were dense enough they would be useless for a human, because we couldn't even get off the ground with them. So we'd end up losing them.

[edit on 3/27/2006 by Zaphod58]