Fig. 2. U-shaped curves of power as a function of flight speed in birds. (A) Estimated mechanical power output required for flight in a European kestrel Falco tinnunculu (from Rayner, 1999). P_aero, total aerodynamic power, P_ind, induced power, P_par, parasite power and P_pro, profile power, V_mp, velocity for minimum power, V_mr, velocity for maximum range. (B) In vivo mechanical power output from wind-tunnel flight across flight speeds as measured using strain gauges, sonomicrometry and wing and body kinematics in dove Zenaida macroura, cockatiel Nymphicus hollandicus, magpie Pica hudsonica (from Tobalske et al., 2003). (C) Oxygen consumption, an index of metabolic power output, measured in cockatiels over a range of flight speeds using gas respirometry (from Bundle et al., 2007).