Relationships of caecal lengths to food habits in North American and other birds

This study elucidates patterns of occurrence and the role of caeca in 166 species of birds having a diversity of food habits. Well-developed caeca occur in granivorous, herbivorous, insectivorous, and some carnivorous (those consuming aquatic invertebrates and some small vertebrates) species whose diets contain large amounts of cellulose or chitin. Poorly-developed or none-existent caeca occur in frugivorous, nectarivorous, piscivorous, and other carnivorous (those consuming carrion, some small vertebrates) species whose diets consist mainly of soluble sugars and proteins. There were no significant age or sex differences in caecal lengths of selected species and caecal lengths are not correlated with body mass. Caecal length does, however, correlate well with small intestine length in species that ferment cellulose and possibly chitin, and with large intestine length in species that are potentially waterstressed and/or consume a low protein diet. Caeca function in digestion of insoluble food and may help conserve critical resources such as water and nitrogen.