Plasma lipid metabolites can provide information about fat metabolism and storage in migrating birds, yet little is known about the influence of diet and nutrition or how they relate to intraindividual differences in body composition of songbirds. We investigated how dietary macronutrient composition affects plasma lipid concentrations and subsequent changes in fat accumulation in white-throated sparrows (Zonotrichia albicollis). Birds fed a low-protein diet with more glucose had higher plasma triglyceride levels and higher average fat mass compared with levels and mass in birds fed diets with less glucose, and birds fed diets with less glucose and more fat had the highest plasma B-hydroxybutyrate concentrations, regardless of protein content. Birds fed the low-protein, high-fat diet also had the highest plasma nonesterified fatty acids. Diet-related changes in plasma triglyceride and B-hydroxybutyrate were more strongly related to fat mass than to lean mass of birds. Nevertheless, diet-related changes in lipid metabolites were more strongly influenced by the intake of certain macronutrients than by body fatness. Thus, plasma lipid metabolites may reliably indicate fat mass and fattening rates of birds, although our results suggest that diet composition must be considered given that certain macronutrients, namely, dietary glucose, may enhance fat deposition in songbirds.