Birds have evolved many unique adaptations to survive, including pneumatic or hollow bones. In fact the skeleton of most backyard birds only comprises about 5% of its total body weight. That means that in some species of birds, their bones are so lightweight that the bird’s feathers weigh more than their entire skeleton.
The bones also contain hollow chambers, or air sacs, that act as a kind of secondary lung system. The air sacs move air through the almost completely rigid lungs in a unidirectional flow instead of the in and out breathing of mammals that mixes old and new air. That means the birds have more oxygen available to them to enhance their efficiency and maintain their normal body temperature