Figure 1.

A conceptual power curve. It is costly to fly slowly as well as fast. In between, the curve has its minimum that corresponds to the minimum power speed (V_mp). If a tangent is drawn from the origin to the curve, the maximum range speed (V_mr) is obtained. If the fuel deposition rate (P_dep) for a bird at stopover sites along the migratory journey is taken into account, the optimal speed is the time minimizing speed (V_mt). In this case, the y-axis is extended downwards to include the fuel-deposition rate. V_mt is found by drawing a tangent from the point of the current deposition rate to the power curve. The intercept between the tangent and the x-axis corresponds to the overall speed of migration (V_migr).