All animals need some kind of locomotion, or ability to move. Birds are lightweight animals that developed the ability to fly in order to adapt to their environment. Flying gave them the ability to escape their predators, as well as make them better hunters. Flying also allows them to escape nasty weather and migrate to warmer temperatures. It's important to note that not all birds can fly -- the ostrich, for example, adapted to its environment by developing the ability to run very fast!