The primary energy source for migration is fat, but nonfat body components can vary in concert with lipid stores in some migrants. The goals of this study were (1) to validate for a small Old World warbler (the blackcap, Sylvia atricapilla) non-destructive methods to measure lean and fat mass, (2) to quantify the relative contribution of lean mass to body-mass change of migrants, and (3) to ascertain what lean tissues might be involved. Using total-body electrical conductivity and dilution space of isotope-labeled water, we measured lean and fat mass with precision of 3%-4% and 10%-15%, respectively. In newly arrived migrants with apparently similar structural size (tarsus length), there was a significant positive correlation between lean mass and fat mass; 37% of each unit change was lean mass and 63% fat. Captive blackcaps, fed ad lib. for 7 d, gained body mass, with 40% being lean mass. When captives were fasted 1.5-3 d, both body mass and lean mass declined; lean mass accounted for 42% of body mass lost. In fasted birds, the masses of liver, stomach, and small intestine declined and accounted for 44% of the total lean mass decline, a disproportionate amount considering that these organs make up only 11% of a blackcap's lean mass. In freshly captured blackcaps, organ masses were positively correlated with lean mass minus the organ masses, suggesting that these organs are a source of lean mass catabolized by migrants. We conclude that migrants' need for protein to rebuild lean mass during stopover could constrain diet selection and require increased foraging time, thus slowing mass gain and lengthening overall migration time. Also, stopover time may be lengthened if time is required to rebuild atrophied organs that are important in food digestion and assimilation.