Metabolic power, mechanical power and efficiency during wind tunnel flight by the European starling Sturnus vulgaris

We trained two starlings (Sturnus vulgaris) to fly in a wind tunnel whilst wearing respirometry masks. We measured the metabolic power (P_met) from the rates of oxygen consumption and carbon dioxide production and calculated the mechanical power (P_mech) from two aerodynamic models using wingbeat kinematics measured by high-speed cinematography. P_met increased from 10.4 to 14.9 W as flight speed was increased from 6.3 to 14.4 m s^–1 and was compatible with the U-shaped power/speed curve predicted by the aerodynamic models. Flight muscle efficiency varied between 0.13 and 0.23 depending upon the bird, the flight speed and the aerodynamic model used to calculate P_mech. P_met during flight is often estimated by extrapolation from the mechanical power predicted by aerodynamic models by dividing P_mech by a flight muscle efficiency of 0.23 and adding the costs of basal metabolism, circulation and respiration. This method would underestimate measured P_met by 15–25 % in our birds. The mean discrepancy between measured and predicted P_met could be reduced to 0.1±1.5 % if flight muscle efficiency was altered to a value of 0.18. A flight muscle efficiency of 0.18 rather than 0.23 should be used to calculate the flight costs of birds in the size range of starlings (approximately 0.1 kg) if P_met is calculated from P_mech derived from aerodynamic models.