Bird Slow Flapping Diagram
Flapping flight consists of two distinct movements: the power stroke and the back stroke. In the power stroke, the wings move forward and down; the back stroke returns the wings to the position from which the next power stroke will commence. During a glide the inner wings (the secondary feathers) and the "hand " regions (the primary feathers) are spread to produce a continuous airfoil. During a flap, however, the two parts of the wing carry out different functions. The inner wings give lift, while the "hands" pull the body forward. During the power stroke the primary feathers are linked together to produce a near-perfect airfoil, giving the maximum thrust and minimum drag.