Metabolic Routing of Dietary Nutrients in Birds: Effects of Dietary Lipid Concentration on Δ13c of Depot Fat and Its Ecological Implications

A st act.—During migration, many songbirds eat primarily fruit while depositing fat. Given that some fruits contain mostly carbohydrate and others contain mostly lipid, the ability of birds to fatt en may depend on the macronutrient composition of the fruit. Stable isotopes of carbon may be useful in determining the source of nutrients used for synthesizing fat, because the enzyme that regulates the transfer of carbon skeletons from carbohydrate into fat synthesis has a higher affi nity for 12C than for 13C, whereas dietary lipids can be directly incorporated into animal fat. Thus, fat stores of animals that are synthesized directly from dietary lipid should have isotopic signatures similar to dietary lipid, whereas biosynthesis of fats from dietary carbohydrates should produce changes in isotopic signatures. We tested these predictions by manipulating the concentrations and isotopic signatures of macronutrients in diets fed to Yellow-rumped Warblers (Dendroica coronata). The δ13C of depot fat in birds fed high-lipid diets was similar to that of dietary lipid, whereas δ13C of depot fat in birds fed low-lipid diets indicated that a combination of dietary lipid and carbohydrate were used to synthesize depot fat. Models that incorporated 8‰ discrimination between dietary carbohydrate and depot fat consistently estimated the proportion of dietary lipid and carbohydrate routed into depot fat. Stable-isotope analysis of macronutrients in the diet of wild birds combined with estimates of the eff ects of diet composition on the isotopic signature of depot fat in birds off er a method to identify the relative importance of nutritional resources used by songbirds to deposit fat. Received 18 May 2005, accepted 6 August 2006.