Why don't all birds fly?

We know that birds have wings, but do all birds fly? Surprisingly, the answer is "no!"

Birds such as the ostrich, the emu and the kiwi do not soar through the sky. These birds have a breastbone (or sternum) that is smooth. There is no ridge, or keel, on this breastbone.

The keel is where the flight muscles of flying birds are attached. Birds that don't fly certainly don't need a keel! Birds that have a smooth, keel-free breastbone are called ratites. As it turns out, ratites don't need to fly! For example, the kiwi bird of New Zealand has strong legs that allow it to run faster than a human! Emus are also fast runners, and when they feel threatened by a predator, they can kick with their large feet.