BASAL METABOLIC-RATE AND THE MASS OF TISSUES DIFFERING IN METABOLIC SCOPE - MIGRATION-RELATED COVARIATION BETWEEN INDIVIDUAL KNOTS CALIDRIS-CANUTUS

To examine whether variability in the basal metabolic rate (BMR) of mi grant shorebirds is a function of a variably sized metabolic machinery or of temporal changes in metabolic intensities at the tissue level, BMR, body composition and activity of cytochrome-c oxidase (CCO, a mar ker for maximum tissue respiration) were measured in 14 captive Knots Calidris canutus islandica in late spring, during the period of mass l oss after the migratory body mass peak. Although the body mass cycle o f captive birds closely followed the changes of free-living conspecifi cs, their fat-free mass of muscles and organs was somewhat lower and t heir fat content higher. BMR significantly declined during mass loss, as did the fat-free dry mass. BMR was an allometric function of both b ody mass (exponent=0.687) and lean dry mass (exponent=1.132). Fat-free dry mass of heart sind flight muscle decreased with the loss of fat. CCO-activity was determined in heart, flight muscle, leg muscle, liver and kidney. It was highest in heart and flight muscle and low in the other tissues. CCO-activity was not correlated with total fat mass. In traspecific migration-related variation in BMR seems better explained by variation in the mass of organs with a high metabolic scope (as ind icated by high CCO-activity), than by variation in the intensity of ti ssue metabolism.