Body size establishes the scaling of avian postnatal metabolic rate: an interspecific analysis using phylogenetically independent contrasts

The avian postnatal metabolic rate literature is reviewed using power equations, Y = aM b , to describe the relation between postnatal resting metabolic rate (RMR) and chick body mass (M) for 25 species. In altricial species, the relation between RMR and M from hatching to fledging can be described by a single power equation, whereas in most nonaltricial species two such equations are needed, one for chicks weighing less than about 25% of mature mass ( M a ) and a second for larger chicks. For altricial chicks and larger nonaltricial chicks, the body‐mass exponent, b, of 25 intraspecific power equations ranged from 0.25 to 1.67 and varied inversely with M a . The scaling of postnatal RMR is thus unlike that of either adult or hatchling metabolism in that it is size dependent. We examined the relationship between intraspecific b and M a using Felsenstein's independent contrasts method to control for statistical complications due to the hierarchical nature of phylogenetic relationships. This “phylogenetic regression” technique yielded the relation b = 1.6 M a ‐015 , in which mature mass explained 38% of the variation in b. The mass exponent of this equation (‐0.15) did not differ significantly from that determined by nonphylogenetic methods (‐0.17). In altricial chicks and larger nonaltricial chicks, the scaling coefficient, a, of the interspecific power equations varied with adult mass according to the phylogenetically determined relation a (kj/h) = 0.0052M a 0.65 and was higher in fed than in fasted chicks. Equations derived in this analysis permit one to estimate the RMR of a growing chick from its mass and adult body mass and provide a basis for evolutionary and ecological comparisons.