Fig. 6. Mean vertical and horizontal accelerations during downstroke and upstroke resulting from aerodynamic forces plotted versus flight speed. The values shown are means ± 1 S.D. for the four birds. Vertical and horizontal upstroke and downstroke accelerations differed significantly between stroke phases (P<0.001, paired t-test), and vertical accelerations differed significantly across speeds. Note that maintaining position in the wind tunnel requires that aerodynamic forces produce a mean vertical acceleration of +9.81 m s^–2 (to counter gravity) and a horizontal acceleration of 0 m s^–2; there was a slight tendency toward forward acceleration in the cockatiels, especially at faster flight speeds.