“Some birds are not meant to be caged, that's all. Their feathers are too bright, their songs too sweet and wild. So you let them go, or when you open the cage to feed them they somehow fly out past you. And the part of you that knows it was wrong to imprison them in the first place rejoices, but still, the place where you live is that much more drab and empty for their departure.”
 ― Stephen King, Rita Hayworth and Shawshank Redemption: A Story from Different Seasons

As the bird flaps its wings using the strong breast muscles to fly, the feathers on the wings act like propellers and provide thrust for flying. The bird moves forward and begins flapping his or her wings. During this flapping, the pressure created over the top of the wing is lower than the pressure created under the wing. This pressure difference results in lift. Similarly, when they want to land, they spread their tails, lower their feet, and push their bodies onto the ground. So, this was all about lifting off, thrusting, and landing.

- http://www.buzzle.com/articles/how-do-birds-fly.html