Birds fly by flapping their wings up and down as well as forward through the air. The vertical motion of the wing is greatest farther away from the body. Closer to the body, wing movement is primarily horizontal.
Bird wings possess both curve and angle. The curve at the edge helps to reduce air resistance, while the slight angle directs air downward. The downward force of the air creates an equal and opposite force that pushes upward on the wing, producing lift. Birds constantly adjust the angle of their wings while flying to produce the right amount of lift in the right direction. It is also important that the angle be correct along the entire length of the wing. The part of the wing farthest from the bird's body has steeper vertical motion, which means that the wing must twist along its length as the bird flies to maintain appropriate angles. The twisting of the wing creates additional force rearward, moving the bird forward. All of this occurs during the downstroke. During the upstroke, birds take measures to avoid drag. Birds reduce the angle of their wing to zero during the upstroke to decrease air resistance. They also partially fold their wings during an upstroke, which reduces the length of their wingspan. This reduces the drag from the outer feathers.