MECHANICAL POWER OUTPUT OF BIRD FLIGHT

Aerodynamic theory predicts that the power required for an animal to f ly over a range of speeds is represented by a 'U'-shaped curve, with t he greatest power required at the slowest and fastest speeds, and mini mum power at an intermediate speed(1-6) Tests of these predictions, ba sed on oxygen consumption measurements of-metabolic power in birds(7-1 2) and insects(13), support a different interpretation, generating eit her flat or 'J'-shaped power profiles, implying little additional dema nd between hovering and intermediate flight speeds(14). However, respi rometric techniques represent only an indirect assessment of the mecha nical power requirements of flight and no previous avian study has inv estigated an animal's full range of attainable level flight speeds. He re we present data from in vivo bone-strain measurements of pectoralis muscle force coupled with wing kinematics in blackballed magpies (Pic a pica), which we use to calculate mechanical power directly. As these birds flew over their full range of speeds, we offer a complete profi le of mechanical power output during level flapping flight for this sp ecies, Values of mechanical power output are statistically indistingui shable (that is, the power curve is flat) over most forward-flight spe eds but are significantly higher during hovering and night at very low speeds.