AN EXPERIMENTAL AND COMPARATIVE-STUDY OF DIETARY MODULATION OF INTESTINAL ENZYMES IN EUROPEAN STARLINGS (STURNUS-VULGARIS)

European starlings (Sturnus vulgaris) are omnivorous passerine birds that include significant amounts of starchy grains (e.g., oats and barley) in their diet when insects and fruit are not available. We predicted that starlings would have high levels of intestinal maltase activity (the main enzyme involved in the last step of the digestion of complex carbohydrates) and the ability to up-regulate intestinal hydrolases (maltase, isomaltase, and aminopeptidase-N) in response to changes in nutrient intake. Captive starlings were fed three diets: a diet containing 52.5% cornstarch, a carbohydrate-free diet, and an insect diet. Diet had a significant effect on intestinal morphology: birds fed the carbohydrate-free diet had significantly longer intestines and larger intestinal areas than those fed either of the two other diets. Diet had a significant effect on aminopeptidase-N and isomaltase activity. Both aminopeptidase-N and isomaltase increased with protein and carbohydrate intake, respectively, but the magnitude of increase was small. Diet had no effect on maltase activity per unit intestinal area, however. Surprisingly, total maltase activity was highest in birds fed the carbohydrate-free diet, perhaps because of increased intestinal area in these birds. We used standardized phylogenetic contrasts to compare maltase activity in starlings with that of closely related insectivorous and frugivorous species. Starlings had maltase activities that were not significantly different from those of close relatives that do