System of coiled tubes (the guts) leading from mouth to anus, through which food passes during the process of digestion. In birds it consists of the bill (see 'bill') and mouth, oesophagus (or gullet) and crop, proventriculus and gizzard (corresponding to the mammalian stomach), small intestine, large intestine and caeca, and the cloaca (combination of anus and urinary plus genital openings). The development of the different parts differs greatly in relation to feeding habits; e.g. the crop may be rudimentary or absent and the gizzard much reduced. Since little or no mechanical breakdown is performed by the mouth or bill, it is capable of a wide gape and the pharynx region (throat) is widely distensible. The tongue is usually large and pointed with an axial stiffening rod of bone (hyoid). The crop is a distensible sac-like extension of the gullet and has a single opening connecting it with the lumen of the oesophagus. The relative size is highly variable between species, depending on feeding habits; e.g. highly developed in grain-eating birds and rudimentary in several other groups. The primary function is that of food storage, especially during the breeding season for the young, when in many species the food undergoes some digestion in the crop before being offered to the young. The proventriculus amplifies the peristaltic waves initiated in the crop and passes the moist undigested food, together with secreted digestive juices, into the gizzard (the 'real' stomach). The gizzard is a very muscular stomach with a corrugated, leathery lining. The food is ground here with help from grit and small stones occasionally eaten by, especially grain-eating, birds. Frugivorous (fruit-eating) and carnivorous species have less developed gizzards. In the small intestine the real enzymatic digestion takes place with the help of secretions from liver and pancreas. Where the small intestine runs into the large intestine, two blind tubes branch off, the caeca. Here, in herbivorous birds, the food is further digested by the agency of bacteria (and enzymes); in carnivorous birds and parrots the caeca are lost.

Alternative forms for alimentary system : caecum, cloaca, gizzard, gullet, proventriculus.