PODLESAK, D. W.*; MCWILLIAMS, S. R.; Univ. of Utah; Univ. of Rhode Island: Birds metabolically route exogenous nutrients to rebuild digestive organs after fasting

Many songbirds catabolize their digestive organs and other proteinaceous tissues while fasting during migration. The gut-limitation hypothesis proposes that after arrival at a stopover site songbirds must rebuild digestive organs prior to replenishing other proteinaceous tissues that were catabolized during flight. Birds may route macronutrients from the diet or from endogenous sources to rebuild the digestive organs. We investigated how macronutrients in diet and the bird were allocated to digestive organs in yellow-rumped warblers, (Dendroica coronata), after fasting by comparing tissue mass and carbon and nitrogen isotopic values of multiple proteinaceous tissues from fasted birds and from birds that were fasted and then refed one of two diets with different macronutrients. Birds metabolically routed carbon and nitrogen primarily from the diet to rebuild the digestive organs after fasting, and within 48hrs after the end of the fast, birds were also rebuilding the pectoralis muscle from dietary carbon and nitrogen. Thus, our data support the hypothesis that birds utilize exogenous macronutrients to rebuild the digestive organs after fasting, but our data do not support the prioritized rebuilding of protein stores as proposed by the gut-limitation hypothesis