|MadSci Network: Zoology|
There are two things you need to look at to figure this out. First, compare the feathers on wings of birds that can fly with the wings on birds that don't (a penguin would be a good example of a bird with wings that doesn't fly). To be able to fly one of the things a bird needs is special types of feathers. Surpisingly enough these feathers are called flight feathers. The other part of the bird you need to look at is the skeleton. The specific part of the skeleton you need to look at is the breastbone. In all birds that fly the breastbone has a large ridge (called a keel) down the center of the breastbone. Birds that don't fly, like ostriches and emus, don't have this keel. However, a penguin will have this keel because it still uses its wings like it's flying when it swims. The keel is necessary to attach the large muscles that are required to flap the wings. Even birds that don't fly very much (like chickens) still have the keel and the large muscles attached to them.This is why you find so much more meat on a chicken breast than you do on the other pieces. If you cooked an ostrich the most meat would be on the legs and thighs instead of the breast because an ostrich doesn't have the large muscles needed for flying. There are other less obvious parts of a bird that have characteristics that are necessary for flying. However, the breast muscles and types of feathers are probably the most important and easiest to see.
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