THE TRADE-OFFS BETWEEN DIGESTION RATE AND EFFICIENCY IN WARBLERS AND THEIR ECOLOGICAL IMPLICATIONS

Frugivory in birds is associated with rapid gut passage whereas insectivory is associated with slower gut passage. This is interpreted by some avian ecologists to reflect an inherent digestive constraint on diet selection, but it could also result from dietary acclimation. We predicted that Yellow-rumped Warblers (Dendroica coronata) acclimated to fruit-, insect-, and seed-based diets would exhibit retention times that increase in that rank order, because this is the rank order of retention time across species that eat these types of food. We also predicted a trade-off between rate of processing (the inverse of retention time) and extraction efficiency. This is based on the assumption that digestive enzymes or absorptive capacity occur in the gastrointestinal tract at levels that are not in great excess, and so less contact time between enzymes and digesta reduces the extraction efficiency. To test these predictions, we measured retention time (using aqueous and lipid-phase inert markers) and extraction efficiency of glucose, sucrose, leucine, starch, and the lipid glycerol trioleate (using radio-labeled nutrients and inert markers). Our results were in accordance with predictions. Mouth-to-anus total mean retention time (TMRT) of Yellow-rumped Warblers acclimated to fruit-, insect-, and seed-based diets were, respectively, 46 +/- 9, 62 +/- 6, and 114 +/- 9 min for polyethylene glycol (PEG, an aqueous marker): results were similar for the lipid phase marker in most cases. But Yellow-rumped Warblers that were switched suddenly to an alternate diet did not readjust TMRT when tested 2 h later or did so incompletely. We found no diet-related morphological changes in the digestive tract, and thus attribute these results to changes in gut motility. Extraction efficiency was uniformly high across all diets for glucose (88 +/- 1%), but varied among diet groups according to our prediction for leucine (range 82 - 94%), sucrose (58 - 85%), starch (9 - 48%), and lipid (18 - 82%). We review how features of the Yellow-rumped Warbler's digestive system permit relatively high energy uptake across a wide variety of food types. The most notable constraining feature was a low starch hydrolysis rate, probably restricting them from relying on starchy foods. Thus, digestive strategy (i.e., a combination of retention time of food in the gut and digestive efficiency) somewhat determines diet, but in more respects diet determines strategy.