Question

A birds bones are filled with...

Answer 1

Answer:

But instead of being filled with marrow as your bones are, a hollow bone has an air cavity inside. The hollow part of a bird bone isn't wasted space. In some bones, the hollow cavities contain extensions of the air sacs from the lungs. These air sacs help the bird to get the oxygen it needs to fly quickly and easily.

Answer 2

Explanation:

A birds bones are filled with air ...