Fig. 7. Mean inertial work (A) and inertial power (B) associated with wing acceleration during upstroke and downstroke plotted versus flight speed. Note that while inertial work in upstroke is less than in downstroke at all speeds, this is not the case for inertial power. The reduced duration of upstroke at slower flight speeds increases the upstroke inertial power to the point where it is nearly equal to the downstroke power. As we found previously in cockatiels (Hedrick et al., 2003), changes in wingbeat duration were entirely due to changes in upstroke duration, as downstroke duration did not vary.