removing the effect of body mass, RMR and MMR show similar scaling exponents [24]. It is unclear whether this relationship will hold for the wider range of species used in this study, since the Rezende et al, [24] analysis was dominated by data for passerines. The relationship does appear to hold ontogenetically for at least one species, however, the Australian Brush Turkey [39]. The relationship between RMR and flight MR, however, has not been extensively examined in birds, with the exception of some attempts to estimate the relationship [40,41]. Therefore, while we must interpret our results carefully, it seems that RMR may be an adequate proxy for maximum metabolic demand. A combination of the small RMR dataset that is available for walking species and the diversity within this group (a mixture of flightless and ground dwelling birds) may explain why there is no association between uncinate process length, body size, and RMR in these birds. 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