Fig. 4. (A) The relationship between mechanical power output and flight speed expressed as inter-individual means ± S.D. (N=5 for each speed; mean and minimum wingbeat sample sizes were 46 and 17, respectively, across all individuals). Pectoralis force recordings were calibrated using an aerodynamic power analysis at two intermediate flight speeds (7 m s^-1 and 9 m s^-1). The broken lines indicate the possible range of variation in muscle power output given varying aerodynamic assumptions. (B) A power curve for an individual cockatiel showing within-individual means ± S.D. (mean and minimum sample sizes were 50 and 19 wingbeats, respectively, per animal for each speed). The cockatiels tended to repeatedly gain and lose potential energy while flying in the wind tunnel, leading to large variation in per-wingbeat power output at all but the fastest speeds. We restricted our analysis to sequences of wingbeats with no net change in potential energy but allowed individual wingbeats that resulted in a change in potential energy.

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