Fig. 3. Whole wingbeat in vivo work loops obtained from an individual cockatiel flying at (A) 1 m s^-1, (B) 7 m s^-1 and (C) 13 m s^-1. The direction of each loop is counter-clockwise, resulting in positive work. Pectoralis mass-specific power output, wingbeat duration and work `shape factor' are noted in the upper left for each loop. The shape factor quantifies work loop shape by dividing the actual loop area by the theoretical maximum area for the observed peak force and length change.