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In the calidrine sandpiper red knot (Calidris canutus), the weeks preceding takeoff for long-distance migration are characterized by a rapid increase in body mass, largely made up of fat but also including a significant proportion of lean tissue. Before takeoff, the pectoral muscles are known to hypertrophy in preparation for endurance flight without any specific training. Because birds facing cold environments counterbalance heat loss through shivering thermogenesis, and since pectoral muscles represent a large proportion of avian body mass, we asked the question whether muscle hypertrophy in preparation for long-distance endurance flight would induce improvements in thermogenic capacity. We acclimated red knots to different controlled thermal environments: 26[degrees]C, 5[degrees]C, and variable conditions tracking outdoor temperatures. We then studied within-individual variations in body mass, pectoral muscle size (measured by ultrasound), and metabolic parameters [basal metabolic rate (BMR) and summit metabolic rate ([M.sub.sum])] throughout a 3-mo period enclosing the migratory gain and loss of mass. The gain in body mass during the fattening period was associated with increases in pectoral muscle thickness and thermogenic capacity independent of thermal acclimation. Regardless of their thermal treatment, birds showing the largest increases in body mass also exhibited the largest increases in [M.sub.sum.]. We conclude that migratory fattening is accompanied by thermoregulatory side effects. The gain of body mass and muscle hypertrophy improve thermogenic capacity independent of thermal acclimation in this species. Whether this represents an ecological advantage depends on the ambient temperature at the time of fattening. muscle mass; summit metabolic rate; basal metabolic rate; thermogenic capacity; phenotypic flexibility

Visser, G. Henk, Anne Dekinga, Bart Achterkamp, and Theunis Piersma. Ingested water equilibrates isotopically with the body water pool of a shorebird with unrivaled water fluxes. Am J Physiol Regulatory Integrative Comp Physiol 279: R1795-R1804, 2000.--We investigated the applicability of [sup.2]H to measure the amount of body water (TBW) and water fluxes in relation to diet type and level of food intake in a mollusk-eating shorebird, the Red Knot (Calidris canutus). Six birds were exposed to eight experimental indoor conditions. Average fractional [sup.2]H turnover rates ranged between 0.182 [day.sup.-1] (SD = 0.0219) for fasting birds and 7.759 [day.sup.-1] (SD = 0.4535) for birds feeding on cockles (Cerastoderma edule). Average TBW estimates obtained with the plateau method were within the narrow range of 75.985.4 g (or between 64.6 and 70.1% of the body mass). Those obtained with the extrapolation method showed strong day-to-day variations (range 55.7-83.7 g, or between 49.7 and 65.5%). Average difference between the two calculation methods ranged between 0.6% and 36.3%, and this difference was strongly negatively correlated with water flux rate. Average water influx rates ranged between 15.5 g/day (fasting) and 624.5 g/day (feeding on cockles). The latter value is at 26.6 times the allometrically predicted value and is the highest reported to date. Differences in [sup.2]H concentrations between the blood and feces (i.e., biological fractionation) were small but significant (-3.4% when fed a pellet diet, and - 1.1% for all the other diets), and did not relate to the rate of water flux ([[Chi].sup.2].sub.1] = 0.058, P [is less than] 0.81). We conclude that the ingested water equilibrated rapidly with the body water pool even in an avian species that shows record water flux rates when living on ingested marine bivalves. stable isotopes; water fluxes; evaporative water loss; isotope kinetics