Digestive adjustments in cedar waxwings to high feeding rate

Abstract

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 capacity for breakdown and absorption of nutrients will increase, and (c) the mechanism responsible for the increase in total capacity will be an increase in amount of intestine rather than an increase in intestinal tissue-specific enzyme activity or nutrient transporter activity. We measured gut anatomy, retention time of digesta, enzyme hydrolysis rates, nutrient absorption rates, and digestive efficiency in individual cedar waxwings (Bombycilla cedrorum) acclimated to –20°C or +21°C. Compared with cedar waxwings held at +21°C, waxwings acclimated to –20°C more than tripled their daily food intake. Mass of digestive organs increased by 22–53%, but rates of enzyme activity and nutrient uptake per unit of small intestine did not change significantly. Retention time of digesta declined slightly, and there was a small decrease in digestive efficiency. As predicted, the main adjustment to increased energy requirements and food intake was 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 increase their gut size (i.e., that the increase in gut size was maximal). If adjustments involving gut size require weeks of acclimation time, migration patterns and the pace of migration in birds could be influenced by time required for preparation of the gut. J. Exp. Zool. 283:394–407, 1999. © 1999 Wiley-Liss, Inc.