A study of the gross and microscopic anatomy of the caeca of Japanese quail (Coturnix coturnix) and spruce grouse (Canachites canadensis) revealed paired organs that, in combined length, were, respectively, about 35 and 76% of the length of the small intestine, internally, the caeca showed a zonation that reflected differences in lumen size, length of villi, and degree of development of internal folds. The columnar epithelium, lining the caeca, possessed both an absorptive capacity through the presence of microvilli and a secretory capacity through the presence of epithelial glands.Changes in lengths of the caeca in Japanese quail were related to changes in rates of food consumption and not to food quality. Larger volumes of food eaten per day, under colder ambient temperatures, produced significant increases in caecal lengths.We discuss the adaptive significance of the caeca and propose that their major function is to permit a sorting of the intestinal contents: bulky, largely indigestible cellulose material is readily discarded from the intestine and the nutrient-rich residue is retained in the caeca for longer periods of time, where further digestion and absorption probably occur.