ABSTRACT At air speeds approximating to those of equilibrium gliding (14–15 m/sec), flapping by Andean condors acts to reduce sinking speed and does so without increasing forward air speed. Apparently the flapping wing provides lift by increasing the speed of the air striking its surfaces and by twisting at the same time so as to present airfoils with optimum orientation toward the changing direction of the relative wind throughout the cycle. Positive or negative components of lift and thrust forces are produced in various combinations depending on the angle of attack. Thrust forces appear at reduced air speeds, a normal condition in take off. In level flight the average velocity of the wind around the wing is greater during the downstroke than during the upstroke because of the anterior rotation of the downstroke axis. Mechanical efficiency of the lever system operating in a wingbeat is greatest during the downstroke so this orientation is important for producing maximum lift forces. The minimum power output generated by a startled condor in escape flight was computed to be 54 W/kg muscle. Assuming (a) the ratio of power output for exceptional and sustained efforts by human muscle (40:17) is similar to that of the condor and (b) the mechanical efficiency for exceptional and sustained efforts is similar, then approximately 23 W/kg wing muscle are available, only about 70% of that estimated as necessary for sustained flight in still air. On the basis of flight performance and of differences in physical dimensions the turkey vulture appears to be capable of more extended flights in still air than the condor.