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Many birds exhibit seasonal switches in diet and thus alter nutrients predominating their food intake. We tested for dietary modulation of pancreatic enzymes in Yellow-rumped Warblers (Dendroica coronata), a species for which such dietary changes are documented. Birds were fed three diets formulated from either fruit, insect, or seed. We predicted that pancreatic carbohydrase amylase, and the proteases trypsin and chymotrypsin, would be modulated in direct correlation with relative levels of dietary carbohydrate and protein, respectively. In contrast to the prediction, there was no significant dietary effect on activities of amylase and chymotrypsin, and a significant change in trypsin activity (highest on seed diet) occurred in the wrong direction with regard to the prediction. Considering the statistical power of our tests, we conclude that Yellow-rumped Warblers represent a case of only modest ability, at best, to modulate pancreatic enzymes. For some diets, especially high-starch diets that they digest relatively poorly and on which they tend to lose body mass, they may lack sufficient enzyme and hence are constrained in their dietary niche.

Is Diet-shifting Facilitated by Modulation of Pancreatic Enzymes? Test of an Adaptational Hypothesis in Yellow-rumped Warblers