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mechanical power required for flight in birds can be calculated at different airspeeds according to a simple theory (Pennycuick, 1975; Pennycuick, 1989). For cruising flapping flight, three main power components sum to give the total mechanical power: induced power arising from the rate of work required to support the weight of the bird; profile power required to overcome the drag of the flapping wings; and parasite power required to overcome the drag of the body. Parasite drag depends on both skin friction and the pressure drag caused by the body. The skin friction drag component arises as a result of the body surface roughness and the associated viscous shearing forces tangential to the surface, while the parasite (or form) drag results from the distribution of pressure normal to the surface (e.g. McCormick, 1995). The different components of drag responsible for any of the three power components can b