Re: Why can some birds fly and others can't?

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.