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.

Flexibility in flight behaviour of barn swallows (Hirundo rustica) and house martins (Delichon urbica) tested in a wind tunnel.

The flight behaviour of barn swallows (Hirundo rustica) and house martins (Delichon urbica) was tested in a wind tunnel at 15 combinations of flight angles and speeds. In contrast to that of most other small passerines, the intermittent flight of hirundines rarely consists of regular patterns of flapping and rest phases. To vary mechanical power output, both species used intermittent flight, controlling the number of single, pulse-like wingbeats per unit time. House martins in descent tended to concentrate their wingbeats into bursts and performed true gliding flight during rest phases. Barn swallows mainly performed partial bounds during brief interruptions of upstrokes, which they progressively prolonged with decreasing flight angle. Thus, identification of distinct flapping phases to calculate wingbeat frequencies was not feasible. Instead, an effective wingbeat frequency for flight intervals of 20 s, including partial bounds, was introduced. The effective wingbeat frequencies of house martins (N=3) ranged from 2 to 10.5 s(-1), those of barn swallows (N=4) from 2.5 to 8.5 s(-1). In both hirundine species, effective wingbeat frequency was found to decrease almost linearly with decreasing flight angle. With changes in air speed, wingbeat frequency varied according to a U-shaped curve, suggesting a minimum power speed of roughly 9 m s(-1). The duration of the down- and upstrokes varied systematically depending on flight angle and air speed.