American robins (Turdus migratorius) switch from eating fruits in the fall to insects in the spring. Our objective was to determine the physiological and morphological changes associated with such a switch. Three nonexclusive hypotheses addressed possible mechanisms operating on different levels. First, we hypothesized that birds on the two diets would differ in intestinal absorption rates of sugars and amino acids. We predicted that individuals on a high-protein low-carbohydrate insect diet would exhibit higher proline and lower glucose absorption than individuals on a low-protein high-carbohydrate fruit-based diet. Uptake rates of glucose and proline were measured in vitro, using an everted sleeve technique. We found no significant differences in uptake between the two groups and so rejected the hypothesis. The second hypothesis, that retention time of digesta in the gut changes with diet, was tested using an inert marker (polyethylene glycol) injected into insects or fruit and fed to birds. We predicted and found that the marker was excreted significantly faster in birds eating fruit rather than insects. Thus this hypothesis is supported. The third hypothesis focused on changes in gut morphology as a basis for shifts in digestive processing. We found no differences in gut length, nominal surface area, or volume and so rejected this hypothesis. Note that hypotheses two and three are closely related, because retention time is proportional to gut volume/digesta flow. Because gut volume did not differ between the two groups, the difference in retention time must have been due to a change in flow. Taken together, these results suggest that short retention times are likely an important adaptation to frugivory.