1695 Aerodynamic considerations of bird flight suggest that, for geometrically similar animals, the biomechanical power required to fly at the minimum power speed (P min ) is relatively greater for large species than for smaller species, as the biomechanical power required should scale with respect to body mass (M b ) as approximately M b 1.17 (Pennycuick, 1975; Rayner, 1979a). When converting the aerodynamic model predictions for biomechanical power output (P mech ) in watts (W) into estimates for the rate of metabolic energy input (P met ) required to support it (often taken for convenience as equivalent to the rate of oxygen consumed in ml·min –1 ), Pennycuick (1975) assumes that the mechanochemical conversion efficiency of the flight muscles (E fm ) is independent of body mass and recommends the use of a constant value of around 0.23. If E fm is a constant and the ‘central’ cardiovascular adaptations closely reflect the ‘peripheral’ aerobic adaptations of the flight muscles, as predicted by the hypothesis of symmorphosis (Weibel et al., 1991), it would be expected that the cardiovascular systems of large birds that are capable of ‘prolonged’ flight should be adapted to meet the relatively high power required. However, it is clear that the relative rate of blood flow available to the flight muscles of large birds is actually reduced compared to small birds, due to the steady decline in maximal heart rate with increasing body mass (Bishop and Butler, 1995; Bishop, 1997), as in mammals (Weibel et al., 1991). The finding that the maximum heart rate of mammals is primarily an allometric function of body size, regardless of morphological adaptations to sedentary vs athletic locomotor performance such as relative heart mass (Weibel et al., 1991), The Journal of Experimental Biology 208, 1695-1708 Published by The Company of Biologists 2005 doi:10.1242/jeb.01576 When considering the ‘burst’ flight performance of birds, such as during take-off, one of the most important structural variables is the ratio of the mass of the flight muscle myofibrils with respect to body mass. However, when considering ‘prolonged’ flight performance the variable of interest should be the body mass ratio of the mass of the flight muscle myofibrils that can be perfused sustainably with metabolites via the blood supply. The latter variable should be related to blood flow (ml·min –1 ), which in turn has been shown to be a function of heart muscle mass, the value of which is more easily obtainable for different species than that for the mass of perfused muscle. The limited empirical evidence available suggests that for birds and mammals the rate of maximum oxygen consumption scales with heart mass (M h ) as M h 0.88 and that for birds M h scales with body mass (M b ) as M b 0.92 , leading to the conclusion that the rate of maximum oxygen consumption in birds scales with an exponent of around M b 0.82 . A similar exponent would be expected for the rate of maximum oxygen consumption with respect to the flight muscle mass of birds. This suggests that the sustainable power output from the flight muscles may ultimately be limiting the flight performance of very large flying animals, but as a result of circulatory constraints rather than biomechanical considerations of the flight muscles per se. Under the particular circumstances of sustainable flight performance, calculations of rates of metabolic energy consumed by the flight muscles can be compared directly with the estimates of biomechanical power output required, as calculated using various aerodynamic models. The difference between these calculated values for rates of energy input and output from the muscles is equivalent to the ‘apparent’ mechanochemical conversion efficiency. The results of one such analysis, of the maximum sustainable flight performance of migratory birds, leads to the conclusion that the efficiency of the flight muscles appears to scale as M b 0.14 . However, much of this apparent scaling may be an artefact of the application and assumptions of the models. The resolution of this issue is only likely to come from studying bird species at either extreme of the size range. Key words: aerobic flight, muscle efficiency, scaling, heart mass, avian energetics. Summary Introduction Review Circulatory variables and the flight performance of birds Charles M. Bishop School of Biological Sciences, University of Wales Bangor, Bangor, Gwynedd LL57 2UW, UK e-mail: c.bishop@bangor.ac.uk Accepted 8 March 2005 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY