Gut size and the digestion of fibrous diets by tetraonid birds.

A series of diets containing different proportions of heather was fed to red grouse. Heather is the bird's main natural food and contains much fiber; the rest of each diet was a relatively low-fiber, grain-based formulation. With increasing heather and associated fiber contents, food intake increased and overall digestibility of heather was only 9.4%, but, when heather was eaten as the sole food by wild red grouse, its digestibility was 46%. Adaptations to diets containing more heather included increases in the size of the digestive system. Gizzard weight increased linearly with food intake. Lengths of intestines and particularly ceca also increased with intake, but not linearly. Up to an intake of about 50 g dry matter/day no significant change occurred. Above 50 g/day, gut size increased rapidly. Bulk may have started to limit intake at about 50 g/day. One can think of galliform digestion functioning in two modes: 1) a low-fiber mode, in which bulk does not limit intake and fiber digestion is unimportant; and 2) a high-fiber mode, in which bulk limits intake and fiber digestion may well be important. Most studies on avian digestion have been of captive galliforms, most of which probably function in low-fiber mode all the time. Wild galliforms seem to function in high-fiber mode for much of the time, but this has been studied little.