This observation is similar to a question asked in another thread. To further the understanding of how this is possible, I found the following explanation on the world wide web.
"So the shape of the wing and the ability to move it through the air are the two things needed for bird and plane flight. Birds use their strong breast muscles to flap their wings and give them the thrust to move throught the air and fly. In a way, birds use a swimming motion to get the lift needed to fly."