The birds are flying animals but they are also basically bipeds. The theropod dinosaurs, precursors of the birds, were already cursorial bipeds. Because the body structure was modelled by aerodynamical constraints during the evolution, all birds, even those that do not fly anymore, share a typical avian body shape. The osteological differences between birds are more adjustments than deep disruptions. Nevertheless, the birds are very diversified in their way of life and habitat. Yet, the hind limbs of the birds are surprisingly efficient in many manners, such as taking off, landing, swimming and walking. The limb structures adaptability to the various tasks require different mechanical fitness or device such as shock absorber during landing, or thrusters during tacking off. Moreover, almost all birds can walk, even if they have another locomotor specialization, as swimming or flying. Depending on the specialization, the gait features of the walk and the kinematics pattern are slightly modified. The functional adaptability of their hind limb structure may be a key to the evolutive success of the birds.