HOW DOES VARIATION IN BODY-COMPOSITION AFFECT THE BASAL METABOLIC-RATE OF BIRDS

1. Predictions of energy expenditure of individual birds and populatio ns have been made by using allometric equations relating metabolic rat e to body mass. Many birds show seasonal variation in mass and there i s no agreement as to which value of mass to use in such equations. 2. The basal metabolic rates (BMR) of 21 individual Redshank, Tringa teta nus, were measured repeatedly throughout the non-breeding season and r elated 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 indivi dual on its BMR, in order to determine the best predictor to use in in terspecific allometric equations relating the energy expenditure of bi rds that show large seasonal variation in body mass to some component of that mass.4. Between individuals, variation in total lean mass (tot al body mass minus fat mass) was a better predictor than fat mass of v ariation in BMR. 5. Within an individual, however, variation in fat ma ss was as important as variation in total lean mass as a predictor of BMR; in those individuals that showed large seasonal changes in fat ma ss, variation in BMR was best explained by variation in fat mass. Poss ible reasons for this are discussed. 6. As variation in BMR of individ uals is explained to a greater extent by variation in both lean mass a nd fat mass (rather than lean mass alone), we conclude that the best v alue of body mass to use in predictive interspecific allometric equati ons is total body mass and not total lean mass.