4291 Flight is the most energetically demanding, sustained activity that animals perform (Schmidt-Nielsen, 1972; Norberg, 1990). Each of the existing methods for estimating metabolic power during flight (P met ) has some drawbacks as well as advantages. Mask respirometry during flight in a wind tunnel (for references, see Rayner, 1994; Ward et al., 2001, 2002) is the only direct way to measure the rate of gas exchange, from which one can calculate P met . However, this technique has the disadvantage that while being precise, P met for an unencumbered bird is overestimated by 3–30% due to the additional work required to overcome the drag of the respirometry mask and tube (Tucker, 1972; Rothe et al., 1987; Ward et al., 2001, 2002). Doubly labelled water (DLW) is most useful for measuring the cost of long flights (e.g. Wikelski et al., 2003), but cannot resolve short-term variation in flight costs. Monitoring heart rate can provide information on short- term fluctuations in metabolism, but this method requires calibration against respirometry measurements before variation in metabolic rate can be quantified (Butler et al., 1998; Weimerskirch et al., 2001; Ward et al., 2002). P met can also be inferred from the rate of mass loss during flight, which is simple to measure, but produces results that are prone to error since the energy content of mass changes is difficult to assess accurately (Nisbet et al., 1963; Butler et al., 1998; Kvist et al., 1998; Battley et al., 2000). An alternative approach to estimating the energetic cost of flight is to determine mechanical power production for flight (P mech ) from an aerodynamic model (Rayner, 1979a,b; The Journal of Experimental Biology 207, 4291-4298 Published by The Company of Biologists 2004 doi:10.1242/jeb.01281 It is technically demanding to measure the energetic cost of animal flight. Each of the previously available techniques has some disadvantage as well advantages. We compared measurements of the energetic cost of flight in a wind tunnel by four European starlings Sturnus vulgaris made using three independent techniques: heat transfer modelling, doubly labelled water (DLW) and mask respirometry. We based our heat transfer model on thermal images of the surface temperature of the birds and air flow past the body and wings calculated from wing beat kinematics. Metabolic power was not sensitive to uncertainty in the value of efficiency when estimated from heat transfer modelling. A change in the assumed value of whole animal efficiency from 0.19 to 0.07 (the range of estimates in previous studies) only altered metabolic power predicted from heat transfer modelling by 13%. The same change in the assumed value of efficiency would cause a 2.7-fold change in metabolic power if it were predicted from mechanical power. Metabolic power did not differ significantly between measurements made using the three techniques when we assumed an efficiency in the range 0.11–0.19, although the DLW results appeared to form a U-shaped power-speed curve while the heat transfer model and respirometry results increased linearly with speed. This is the first time that techniques for determining metabolic power have been compared using data from the same birds flying under the same conditions. Our data provide reassurance that all the techniques produce similar results and suggest that heat transfer modelling may be a useful method for estimating metabolic rate. Supplementary material available online at http://jeb.biologists.org/cgi/content/full/207/24/4291/DC1 Key words: flight, heat transfer, thermal imaging, thermography, doubly labelled water, metabolic power, bird, efficiency, starling, Sturnus vulgaris. Summary Introduction Metabolic power of European starlings Sturnus vulgaris during flight in a wind tunnel, estimated from heat transfer modelling, doubly labelled water and mask respirometry S. Ward 1, *, U. Möller 2 , J. M. V. Rayner 3 , D. M. Jackson 1,4 , W. Nachtigall 2 and J. R. Speakman 1,4 1 Aberdeen Centre for Energy Regulation and Obesity, School of Biological Sciences, University of Aberdeen, Aberdeen, AB24 2TZ, UK, 2 Institüt der Zoologie, Universität des Saarlandes, D-66041 Saarbrücken, Germany, 3 School of Biology, L. C. Miall Building, University of Leeds, Leeds, LS2 9JT, UK and 4 Rowett Research Institute, Greenburn Road, Bucksburn, Aberdeen, AB21 9SB, UK *Author for correspondence at present address: School of Biology, Bute Medical Buildings, University of St Andrews, St Andrews, Fife, KY16 9TS, UK (e-mail: sw29@st-andrews.ac.uk) Accepted 14 September 2004