Birds are adapted for flight with their hollow bones and their huge chest muscles as well as their feathers that help them to change direction. Birds also have fused bones that make it possible for them to fly with increased flexibility.
The bird's bones are also connected directly to the respiratory system in a unique way that makes it easier to fly. Birds weigh less thanks to their hollow bones so they are able to fly higher and faster. Birds also are able to enlarge their reproductive organs when it is time to mate, and then shrink them during the rest of the year so that they are even more lightweight.