their stomach comes next, with two very different parts. one part is like a straight hollow rod (the "proventriculus") which secretes mild hydrochloric acid and an enzyme to break down protein. the other part of the stomach is the gizzard. this is muscular, and grinds up the contents. in herbivorous birds the gizzard contains some gastroliths (small stones or pieces of grit). bones of fish will mostly be dissolved by the stomach acid. the partly digested and ground-up food now goes to the intestine, where digestion is completed, and most contents are absorbed. anything indigestible, for example remains of feathers, is regurgitated via the mouth, not the cloaca.