Male mallard in mid-flight

A male Mallard in mid-flight. The fundamentals of bird flight are similar to those of aircraft. Lift force is produced by the action of air-flow on the wing, which is an airfoil. This occurs because the air has a lower air pressure just above the wing and higher pressure below. When a bird flaps its wings they continue to develop lift but they also create an additional forward and upward force, thrust, to counteract its weight and drag.