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-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 (D2180). 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 (ef) from an equation based on published data on flight costs in 14 species (body mass 3.8-1,000 g): ef = 17.360M1 013b0 -4236S, 1.926 W where M is body mass (g), b, 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 b, and sw, F, (2,18) = 5.34, P < 0.05]. Received 8 May 1986, accepted 29 November 1986. QUANTIFICATION of the components of daily energy expenditure will improve the understanding of the behavior of free-living birds. This can be achieved by combining time-activity budgets with estimates of the various cost factors (e.g. basal metabolism, thermoregulation, flight; Koplin et al. 1980, Mugaas and King 1981). Energy expenditure during flight is increased greatly relative to during other behaviors (e.g. Berger and Hart 1974). Therefore, even though little time may be devoted to flight, an error in the estimate of the flight cost will produce a considerable error in the total daily energy expenditure estimated from time-activity models. Available allometric equations that predict flight cost in birds are based solely on body mass (e.g. Berger and Hart 1974, Kendeigh et al. 1977, Butler 1980) and provide only a rough estimate of power consumption. Energy expenditure during flight varies among birds of the same mass, according to flight pattern and the aerodynamic and behavioral properties of the species (Nisbet 1967, Utter and LeFebvre 1970, Hails 1979, Dolnik 1982, Flint and Nagy 1984). The Eurasian Kestrel (Falco tinnunculus) has two distinct modes of powered flight: (1) directional flapping flight and (2) wind hovering, which is flapping flight against the wind with zero ground speed (Videler et al. 1983). We determined energy expenditure in kestrels during powered flight. We used this information to quantify total daily energy expenditure of the Eurasian Kestrel throughout its annual cycle (Masman 1986). We used two techniques to determine flight cost. The energy budgets of trained, free-flying kestrels in the laboratory were reconstructed by monitoring daily metabolizable energy intake, oxygen consumption during rest, and time spent flying per day, from which the energy expenditure during directional flight was estimated. In the field we measured daily energy expenditure of free-living kestrels by a doubly labeled water (D2180) technique (Lifson et al. 1955). Time spent flying was recorded simultaneously. Energy expenditure during combined directional flight and wind hovering was estimated from the correlation between daily CO2 production and time spent in flight.

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