How does Variation Body Composition Affect the Basal Metabolic Rates of Birds of Birds?

Abstract

1. Predictions of energy expenditure of individual birds and populations have been made by using allometric equations relating metabolic rate to body mass. Many birds show seasonal variation in mass and there is no agreement as to which value of mass to use in such equations. 2. The basal metabolic rates (BMR) of 21 individual Redshank, Tringa totanus, were measured repeatedly throughout the non-breeding season and related to changes in their body masses and body composition [followed using Total Body Electrical Conductivity (TOBEC)]. 3. We examined the effect of seasonal variation in the amount of fat carried by an individual on its BMR, in order to determine the best predictor to use in interspecific allometric equations relating the energy expenditure of birds that show large seasonal variation in body mass to some component of that mass. 4. Between individuals, variation in total lean mass (total body mass minus fat mass) was a better predictor than fat mass of variation in BMR. 5. Within an individual, however, variation in fat mass was as important as variation in total lean mass as a predictor of BMR; in those individuals that showed large seasonal changes in fat mass, variation in BMR was best explained by variation in fat mass. Possible reasons for this are discussed. 6. As variation in BMR of individuals is explained to a greater extent by variation in both lean mass and fat mass (rather than lean mass alone), we conclude that the best value of body mass to use in predictive interspecific allometric equations is total body mass and not total lean mass.