Is diet shifting facilitated by modulation of intestinal nutrient uptake? Test of an adaptational hypothesis in yellow-rumped warblers.

Yellow-rumped warblers (Dendroica coronata) are one of many avian species that change their diet seasonally. Using them, we tested the predictions that uptake of D-glucose and the amino acid L-leucine will increase as dietary levels of carbohydrate and protein, respectively, are increased and that mediated uptake capacity of the entire small intestine will match nutrient loads from daily food intake. Birds were fed three semisynthetic diets, formulated from fruit, insects, or seeds, for 7 d. Mediated D-glucose uptake in vitro was affected by diet, but contrary to our a priori prediction, fruit eaters eating the diet highest in carbohydrates had the lowest uptake rate. Na(+)-dependent L-leucine uptake at a low concentration (0.01 mmol L-1) was higher in insect and seed eaters, which is consistent with the prediction of adaptation to dietary protein, though dietary fat may also play a role. Mediated D-glucose uptake summed over small intestine length could explain only a small percentage of the estimated whole-animal absorption rate. We thus reject the predictions for carbohydrate but suggest an alternative interpretation of our results that is consistent with economical design.