Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance

Birds during migration must satisfy the high energy and nutrient demands as sociated with repeated, intensive flight while often experiencing unpredict able variation in food supply and food quality. Solutions to such different challenges may often be physiologically incompatible. For example, increas ed food intake and gut size are primarily responsible for satisfying the hi gh energy and nutrient demands associated with migration in birds. However, short-term fasting or food restriction during flight may cause partial atr ophy of the gut that may limit utilization of ingested food energy and nutr ients. We review the evidence available on the effects of long- and short-t erm changes in food quality and quantity on digestive performance in migrat ory birds, and the importance of digestive constraints in limiting the temp o of migration in birds. Another important physiological consequence of fee ding in birds is the effect of diet on body composition dynamics during mig ration. Recent evidence suggests that birds utilize and replenish both prot ein and fat reserves during migration, and diet quality influences the rate of replenishment of both these reserves. We conclude that diet and phenoty pic flexibility in both body composition and the digestive system of migrat ory birds are important in allowing birds to successfully overcome the ofte n-conflicting physiological challenges of migration. (C) 2001 Elsevier Scie nce Inc. All rights reserved.