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(me)t increased from 10.4 to 14.9 W as flight speed was increased from 6.3 to 14.4 ms(-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.