A COMPARISON OF DIGESTIVE EFFICIENCY IN BIRDS OF PREY

Differences in how raptors hunt and what they catch are correlated with size-independent differences in length of the small intestine, the region of the digestive tract responsible for food absorption. The study examined the functional significance of these differences among ten raptor species. Dry matter apparent digestive efficiency was calculated for each species fed a diet of day-old cockerels. For Falconiformes and Strigiformes, efficiencies varied between 75% and 82%, digestive efficiency being positively correlated with intestine length. Generalist species, with a wide prey spectrum and feeding on relatively easily caught prey and carrion, had long intestines and high digestive efficiencies. Raptors specializing on fast-moving, avian prey had short intestines and reduced digestive efficiency. The Peregrine Falco peregrinus and the Common Buzzard Buteo buteo were used as examples of specialist and generalist feeders, respectively. Rabbit and pigeon were fed to both. Buzzards digested both diets more efficiently than Peregrines. Body-mass changes were used to examine the nutritional value of the two diets to each species. Buzzards gained body-mass when eating rabbit, Peregrines lost mass. Both species gained mass when feeding on pigeon. It seems that consideration of diet quality, not just quantity, is essential in understanding raptor food requirements. Generalist raptors have high efficiencies on several diets, specialists compensate for their reduced efficiency by eating food of high nutritional quality. Various aspects of prey quality are examined.