School of Biodiversity, One Health & Veterinary Medicine

Digestive efficiency and feeding strategy in birds and mammals

Meat eating birds and mammals differ in how efficiently they digest food. For example, domestic cats digest their food 10% less efficiently than domestic dogs. This may be linked to the fact that cats have shorter digestive tracts. Studies on wild carnivorous mammals, and meat and fish eating birds, have shown that there is a general relationship between digestive efficiency and gut morphology. As a general rule, short gut species digest food less well than long gut species. This may be linked to the predatory strategy adopted by the animal.

Research has investigated the gut morphology of a wide range of meat and fish eating birds, by post mortem dissection. Various aspects of gut morphology are them considered in relation to the body size of the animal. Feeding trials have also been conducted on the same species of birds and mammals in the field and in captivity to consider the rate of throughput of food and digestive efficiency. This has been done on diets of varying quality.

In meat eating birds there is considerable variation in the length of the digestive tract, with some birds of prey having small intestine lengths only half as long as other species of the same body size. Species with shorter digestive tracts as less efficient in their digestion. There is a good correlation between the hunting strategy adopted by a species and gut morphology and digestive efficiency. Animals which hunt by a rapid chase after prey require a fast acceleration. Species adapted for this method of predation would be favoured by any reduction in the weight of non-muscular parts of the body. In such species there would be selection for reduced gut size if, despite a lower digestive efficiency, this resulted in a greater rate of prey capture. Other species which do not rely on a rapid acceleration to catch prey would be expected to develop the optimum gut length for efficient digestion. These relationships appear to hold well for birds of prey. For example, the species with short guts include those which specialise in catching small passerines in flight. They have reduced digestive efficiency, and as a consequence can only maintain body weight when fed on prey with a high energy content. Species with long guts include scavenging species, and their higher digestive efficiency allows them to eat a wider range of food include low prey of low energy content. Research is continuing into how digestion may be influenced by predation strategy, and the consequences of this for the type of prey selected by predators.

Barton, N.W.H. & Houston, D.C. 1993. A comparison of digestive efficiency in birds of prey. Ibis 135:363-372.