The digestive system of birds exhibits less interspecific variation than that of mammals.

Much variation occurs in the form of the bill, and this is perhaps the counterpart of the dental diversity of mammals (teeth of course being absent from birds, which do not chew their food), but beyond the mouth the relatively minor variation is largely confined to the presence and form of the crop, details of the two-chambered stomach, and the degree of development of the ceca. The digestive organs are relatively small, which contributes to the lightness essential for flight, but highly efficient in rapidly releasing the energy and nutrients from a bird's usually small but frequent meals so that its high metabolic rate may be sustained.