The top of a bird’s wing is curved. In comparison, the bottom of the wing is flat. The path of air splits at the front edge of the wind and meets again at the back of the wing. Thus, air passes under the wing. Since the top of the wing is curved and the bottom of the wing is flat, the air has a greater distance to travel over the wing. Therefore, the air that moves above the wing has to move faster in order to keep up with the air that moves below the wing. Because the air on top of the wing moves faster, there is less pressure above the wing. Likewise, there is greater pressure under the wing where the air is moving slower. Because there is more pressure from the upward-moving air coming from below the wing, the bird moves the bird upward.  This is known as “lift”.