Birds have many adaptations for flight, but three of the most important are feathers, a hollow and highly modified skeleton and internal organs capable of moving ample amounts of oxygen to flight muscles. Flight is very physically demanding, and birds are highly adapted to meet those demands.
The most distinguishing feature of all birds is feathers. Feathers are heavily modified scales that help the wings generate thrust and lift. They also streamline the body to reduce drag as the bird travels through the air.
Since bird bones are hollow, they also reduce weight. Birds have keeled sternum bones for the attachment of enormous muscles that power their wings, and a special bone called the semilunate carpal that allows them to flap their wings efficiently.
All birds have a special system of airways throughout their skeletons and a four-chambered heart. These organ systems provide the flight muscles with a steady supply of oxygen-rich blood that prevents their wings from becoming exhausted while the bird is in the air.