Fig. 1. Characteristic wing orientation at (A) the start of downstroke, (B,C) mid-downstroke, (D) the end of downstroke and (E) mid-upstroke. We judged downstroke to begin when the tips of the primaries were rapidly accelerated by downward angular acceleration beginning at the shoulder, as is visible in the tips of the right wing primaries in A. Mid-downstroke was the moment of greatest angular extent between the two wings. The end of downstroke was judged to occur as just prior to the wrists beginning an upward trajectory. Finally, we considered mid-upstroke to be the frame in which the angle defined by the wrists first reach their maximum upward excursion.