Many bird bones are hollow, filled with thread-like struts made of bone that strengthen them. Their skeletons are also missing many bones that other vertebrates have, such as a lower jaw. They also have groups of bones that are fused together, such as the collar bones, which are fused into the "wishbone". This makes the bones lighter and stronger.

Most birds have a strongly keeled breastbone, to provide attachment for the large wing muscles. Interestingly, most flightless birds, such as the ostrich, do not. Penguins, while flightless, do have the keel, because they use their wings for swimming. Most flightless birds also have solid, not hollow bones.