Abstract
The bird varies the way of walking by physical size. A relatively big bird does bipedalism by right and left legs alternation. There are no big birds by hopping with two pairs of legs together like a sparrow. Generally, the movement of the creature is optimized by the living form, and the physical form changes, too. And the optimal motion is classified with emergency motion and normal motion. The walking gait is the least cost of energy consumtption. From the viewpoint of energy cost, both of the walking of a big bird and the hopping of a small bird should be a movement of the least cost of energy consumption related to their physical structure.
