Bird flight is one of the most complex forms of locomotion in the animal kingdom. Each facet of this type of motion, including hovering, taking off, and landing, involves many complex movements. As different bird species have adapted to specific environments, prey, predators, and other needs, they have developed varied forms of wings and varied forms of flight.
The fundamentals of bird flight are similar to those of aircraft, in which the aerodynamic forces sustaining flight are lift and drag. Lift force is produced by the action of air flow on the wing, which is an air foil. The air foil is shaped such that the air provides a net upward force on the wing, while the movement of air is directed downward. Additional net lift may come from airflow around the bird’s body in some species, especially during intermittent flight while the wings are folded or semi-folded.
Aerodynamic drag is the force opposite to the direction of motion, and hence the source of energy loss in flight. The drag force can be separated into two portions, lift-induced drag, which is the inherent cost of the wing producing lift (this energy ends up primarily in the wingtip vortices, and parasitic drag, including skin friction drag from the friction of air and body surfaces and form drag from the bird’s frontal area. The streamlining of bird’s body and wings reduces these forces.