MORRIS, K.A.*; KARASOV, W.H.; CAVIEDES-VIDAL, E.: Does digestive physiology limit refueling rates in migratory yellow-rumped warblers (Dendroica coronata)?

Smaller guts and slow initial mass gains in migrating birds at stopover sites have led to the idea of a digestive limitation to refueling in migrants. We tested the digestive limitation hypothesis in yellow-rumped warblers using food restriction to simulate infrequent feeding during migration, which may cause a reduction in alimentary tract mass. We predicted that following three days of food restriction (ca. 50% ad libitum intake): 1) digestive organ masses would decrease, 2) digestive capacity as measured by digestive enzyme activities would decrease, and 3) previously restricted birds would not be able to feed and digest at a high rate immediately following return to ad lib. feeding. In our tests of these predictions, restricted birds had small intestine, pancreas, and liver masses 18-22% lower than ad lib.-fed controls. Total activities of sucrase, maltase, aminopeptidase, and amylase were significantly lower (by 37-48%) in restricted birds than in controls, while those of trypsin and chymotrypsin were not. Mass-specific activities did not differ significantly between groups except for aminopeptidase, which was lower in restricted birds. Contrary to our prediction, previously restricted birds were able to feed and digest at a significantly higher rate than the controls immediately following return to ad lib. feeding. Digestive efficiency did not differ between groups. These results suggest that prior to migration yellow-rumped warblers have greater than 20% spare digestive capacity to compensate for declines in their digestive organ masses during migration.