Dietary modulation of intestinal hydrolytic enzymes in yellow-rumped warblers

Many birds exhibit seasonal switches in diet and thus alter the nutrients predominating their food intake. We tested for dietary modulation of small intestine (SI) enzymes in yellow-rumped warblers, a species for which such diet changes are well documented. Birds were fed three diets formulated from either fruit, insect, or seed. We predicted that SI carbohydrases and peptidases would be modulated in direct correlation with relative levels of dietary carbohydrate and protein, respectively. Aminopeptidase N activity was about twice as high in birds eating the highest protein content diet. In contrast, there was no significant dietary effect on any of the carbohydrase activities. There was a proximal-to-distal decrease in activities of all the carbohydrases but not aminopeptidase N. The carbohydrase levels of yellow-rumps are relatively low when compared with other species in the same family and most similar to lower levels found in primarily insectivorous birds rather than in primarily granivorous or nectarivorous species. Considering this and the fact that they do not exhibit dietary modulation of carbohydrase levels, we conclude that yellow-rumps are not highly adapted for handling dietary carbohydrates, especially starch, although they might still efficiently break down and absorb sucrose and maltose if retention time were sufficiently long.