Test of a digestion optimization model: Effects of costs of feeding on digestive parameters

We tested predictions of a chemical reactor model of digestion by manipulating the short-term costs of feeding and then measuring the effect on digestive parameters. We compared residence time of digesta and extraction efficiency of glucose in cold-acclimated waxwings (Bombycilla cedrorum) feeding ad lib, and in birds whose costs of feeding were increased through the addition of intervals of time when they received no food. Such a feeding schedule simulated the ecological situation in which a frugivorous bird like a waxwing encounters food in patches and experiences nonfeeding periods as it searches for new preferred food patches. None of the results were consistent with the predictions of the optimal digestion model: extraction efficiency was independent of costs of feeding, and residence times did not increase as costs of feeding increased. This empirical evidence on the passage of digesta in waxwings suggests that movement of digesta in the guts of birds is much more complex than movement of material in an ideal chemical reactor. Tests of the optimal digestion model have involved manipulating food quality or the costs of feeding, and the conclusions are similar: compensatory modulation of retention time or digesta mixing and not rate of hydrolysis and absorption seem most important in maintaining the remarkably constant digestive efficiency.