The skeleton of an animal provides scaffolding that protects the soft body parts and provides a framework for the legs and wings. Bird bones are light and delicate compared with the bones of most mammals, containing air cavities to help lessen the weight of the bones. The bird skeleton also contains bony braces that provide strength to the light bones, in much the same way that older airplanes have struts on their wings to provide rigidity. Birds have other skeletal differences from other animals. The bird backbone is short and almost fused - this further helps the bird's rigidity during flight.
Birds also have a massive breastbone that is used to anchor the powerful flight muscles.
The most important bird muscles are the massive ones that power bird flight. These muscles are located on either side of the keel of the breastbone, and there are two muscles on each side. As you can see in the diagram on this page, the large outer muscle is called the pectoralis, and it is attached to the innermost long bone of the wing, the humerus. This muscle provides the power for the downbeat of the wing. The other muscle is the supercoracoideus muscle.