Variability in basal metabolic rate of a long-distance migrant shorebird (Red Knot, <i>Calidris canutus</i>) reflects shifts in organ sizes

We studied differences in body composition and basal metabolic rate (BMR, measured in postabsorptive birds under thermoneutral conditions at night) in two subspecies of red knots, Calidris canutus: one that spends the nonbreeding season under energetically costly climatic conditions at temperate latitudes (subspecies islandica in western Europe) and one that winters in the hot and humid tropics (subspecies canutus in West and South Africa). To examine whether the possible differences would be upheld under identical conditions, we kept both groups in captivity as well. Body composition was quantified with respect to the fat and lean components of 10 ''organs'' (breast muscles, leg muscles, stomach, intestine, liver kidneys, lungs, heart, and the skin, and skeleton and attached muscle). Captive birds had lighter lean tissues than wild birds, especially those of the stomach, intestine, kidneys, and liver (the nutritional organs). During the northern winter wild islandica knots had higher lean masses than canutus knots in tropical Africa. Tropically wintering red knots had lower BMRs than their temperate-wintering conspecifics, and birds in long-term captivity had lower BMR values than their free-living counterparts. Average BMR values per category of birds (wild or captive of either subspecies) were strongly correlated with the group averages of lean mass. Prediction of BMR on the basis of total lean mass of red knots undergoing incipient starvation follows this same relationship because metabolically active tissue is being depleted. That the two subspecies converged to similar body composition in captivity indicates that individual red knots may possess considerable flexibility. We argue that red knots, and probably most other long-distance migrants, have metabolic machinery that is able to adjust continuously, depending on the ecological conditions and food types encountered in the course of the year. We further argue that variation in (functional components of) lean mass is the vehicle for seasonal adjustments in metabolic physiology to variable demand levels. Body mass adjustments offer a flexible response enabling red knots to economize on total daily metabolic expenditure whenever conditions allow a relaxation of metabolic scope, such as during winter in the Tropics.