Ecological constraints on digestive physiology in carnivorous and piscivorous birds

Digestion strategies of meat- and fish-eating birds have received little at tention, and the assumption has generally been made that there is rather li ttle variation in digestion parameters between species in these guilds. We show that there is significant though small variation between species in ap parent absorption efficiency. This variation is associated with an apparent trade-off between retention time of digesta and apparent absorption effici ency: short retention times result in low apparent absorption efficiency. W e show that, in raptors, rapid digestion is a consequence of both reduced g ut length and increased flow rate of digesta. We examine the ecological cor relates of digestive strategy in raptors and seabirds. Rapid digestion appe ars to be associated with a pursuit foraging mode, whereas slow digestion t ends to occur in species with a searching foraging mode. We suggest that in raptors which actively pursue aerial prey, the weight savings that can be achieved through rapid digestion exceed the costs in reduced apparent absor ption efficiency. However, a species which adopts a strategy of rapid but i nefficient digestion may be restricted in diet to high-quality food types, whereas species with a slow but efficient digestive strategy are able to ex ploit a wider range of food types, including low-quality prey. J. Exp. Zool . 283:365-376, 1999. (C) 1999 Wiley-Liss, Inc.