This equation relates to still-air migration speed, and wind conditions could greatly influence the actual speed. A graphical illustration of this equation is given in Figure 8.1. For soaring flight, the above equation can be used, but replacing the flapping flight speed (V) with the cross-country soaring-gliding speed (Vcc).

Flight power

Flight power

energy deposition

Figure 8.1 Graphical solution of the theoretical overall migration speed (Vmigr) on the basis of the power required for flight (Pflight), rate of energy deposition at stopovers (Pdep) and flight speed (V). Migration speed is found where a straight line connecting the point on the downward-extended ordinate indicating Pdep to the point in the power-speed plane (V, Pflight) intersects the speed axis. The migration speed can be calculated according to the equation in Box 8.1. From Hedenstrom & Alerstam (1998).

energy deposition

Figure 8.1 Graphical solution of the theoretical overall migration speed (Vmigr) on the basis of the power required for flight (Pflight), rate of energy deposition at stopovers (Pdep) and flight speed (V). Migration speed is found where a straight line connecting the point on the downward-extended ordinate indicating Pdep to the point in the power-speed plane (V, Pflight) intersects the speed axis. The migration speed can be calculated according to the equation in Box 8.1. From Hedenstrom & Alerstam (1998).

Power consumption in gliding flight is generally assumed to be a constant multiple of the basal metabolic rate (BMR), and consequently migration speed will be directly proportional to the cross-country performance (Vmig a Vcc) in soaring migration. The cross-country speed in typical thermal soaring is given as:

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