Mechanical power balance during dynamic soaring cycle (for the same cycle as in Fig. 1c).

The mechanical power in terms of the surplus of the power obtained from the wind over the dissipative drag effect is presented. The mechanical power related to the mass, (solid line), can be divided mainly into two parts: A part showing a high positive power level which is correlated with the upper altitude region where the energy gain from the wind is achieved. This part is opposed by a negative power level which is correlated with the flight phase close to the water surface where the energy loss occurs. The level of negative power is smaller than the one of positive power. The resulting integral power surplus is just sufficient to compensate for the dissipative drag effect. The data of are virtually unbiased. The mechanical power available in flapping flight, (dashed line), is shown for comparison purposes. It is based on the albatross power data described in the Introduction section (maximum lift-to-drag ratio of 20, 8.5 kg mass, and 70 km/h speed). is much smaller in magnitude than . This holds for both the positive and the negative parts of .