A final type of motility is seen in the regurgitation of pellets of bones, hair and feathers from the stomach of raptors. Small Intestine Birds have a small intestine that seems very similar to the small intestine of mammals. A duodenum, jejunum and ileum are defined, although these segments are not as histologically distinct as in mammals. The proximal small intestine receives bile from the liver and digestive enzymes from the pancreas, and the absorptive epithelial cells are decorated with essentially the same battery of enzymes and transporters as in mammals. The large intestine consists of a short colon and, typically, a pair of ceca.
Short villi extend into the lumen of the colon, unlike what is seen in mammals.
The cloaca is an expanded, tubular structure that serves as the common opening of the digestive, reproductive and urinary systems, which opens to the outside of the bird as the vent. As in mammals, the large intestine's primary function is absorption of water and electrolytes. Antiperistalsis that originates in the cloaca is a prominent pattern of motility in the avian colon and has been suggested to assist not only in filling the ceca, but to flush urine from the cloaca into the large gut for absorption of water. In some birds, the ceca appear dispensible and can be removed without apparent harm. In other species, the ceca are important sites for fermentation , and the volatile fatty acids generated from microbial digestion of cellulose contributes significantly to energy demands.