Why do some birds have hollow bones?

Many birds have pneumatic, or air-filled, bones that reduce body weight without making the skeleton weak. Lighter bones help flight, especially in species that need to take off quickly or stay airborne efficiently. In many birds, the air spaces connect with air sacs in the respiratory system, which is part of the highly efficient bird breathing system. Not all bird bones are equally hollow, and flightless birds often have denser bones. This is another example of how bird skeletons are specialized for different lifestyles.