Digestive adjustments in cedar waxwings to high feeding rate

Birds may dramatically increase their food intake during migratory periods or during winter. We tested the hypotheses that when birds are hyperphagic, (a) their digesta retention time and extraction efficiency will not change compared with that of birds feeding at reduced rates, (b) their total capa city for breakdown and absorption of nutrients will increase, and (c) the m echanism responsible for the increase in total capacity will be an increase in amount of intestine rather than an increase in intestinal tissue-specif ic enzyme activity or nutrient transporter activity. We measured gut anatom y, retention time of digesta, enzyme hydrolysis rates, nutrient absorption rates, and digestive efficiency in individual cedar waxwings (Bombycilla ce drorum) acclimated to -20 degrees C or +21 degrees C. Compared with cedar w axwings held at +21 degrees C, waxwings acclimated to -20 degrees C more th an tripled their daily food intake. Mass of digestive organs increased by 2 2-53%, but rates of enzyme activity and nutrient uptake per unit of small i ntestine did not change significantly. Retention time of digesta declined s lightly, and there was a small decrease in digestive efficiency. As predict ed, the main adjustment to increased energy requirements and food intake wa s an increase in gut length, mass, and volume which largely compensated for increased digesta flow at high intake rates. However, we detected a small reduction in retention time and digestive efficiency in waxwings with high intakes which suggests that these waxwings may be unable to further increas e their gut size (i.e., that the increase in gut size was maximal). If adju stments involving gut size require weeks of acclimation time, migration pat terns and the pace of migration in birds could be influenced by time requir ed for preparation of the gut. J. Exp. Zool. 283:394-407, 1999. (C) 1999 Wi ley-Liss, Inc.