Fig. 8. A comparison of the measured cockatiel pectoralis mass-specific muscle power output (black) reported by Tobalske et al. (2003) with three measures of the mass-specific inertial power requirements of flapping flight. The upstroke mass-specific inertial power (red) is the peak wing kinetic energy developed in upstroke divided by the mass of the upstroke musculature and the wingbeat duration. This is the best measure of the muscle power required for upstroke. The downstroke mass-specific inertial power (green) is the peak wing kinetic energy developed in downstroke divided by the pectoralis mass and wingbeat duration. The downstroke excess inertial power is the peak wing kinetic energy in downstroke with the aerodynamic work done during wing deceleration subtracted, i.e. E_k,rd. This sum was converted to a mass-specific power by dividing by the pectoralis mass and wingbeat duration. The muscle masses used to calculate mass-specific powers are given in Table 1.