Energy Expenditure during Free Flight in Trained and Free-Living Eurasian Kestrels (Falco Tinnunculus)

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Abstract
During directional flight trained Eurasian Kestrels (Falco tinnunculus) in the laboratory expended 13.8 W. Free-living birds expended 14.6 W during nonsoaring flight that included both directional flight and wind hovering. The former value was obtained by combining food balance and indirect calorimetry techniques, the latter by doubly labeled water (Dz18O). Because the energy-expenditure rates are so similar for directional flight alone and for wind hovering and directional flight combined, we argue that the value for either mode of kestrel flight may be used to analyze time-energy budgets. We predicted avian flight costs (et) from an equation based on published data on flight costs in 14 species (body mass 3.8-1,000 g): where M is body mass (g), bw is wing span (cm), and Sw is wing area (cm2). Inclusion of the morphological data with body mass significantly improved the prediction of flight cost [r2 = 0.84 vs. r2 = 0.75 without bw and sw, Fc (2,18) = 5.34, P < 0.05]. Received 8 May 1986, accepted 29 November 1986.