Digestive System
Many birds possess a muscular pouch along the esophagus
called a crop. The crop functions to both soften food
and regulate its flow through the system by storing it temporarily. The size and shape of the crop is quite
variable among the birds. Members of the order Columbiformes, such as pigeons, produce a nutritious crop
milk which is fed to their young by regurgitation. Birds possess a
ventriculus, or gizzard, composed of four
muscular bands that rotate and crush food by shifting the food from one area to the next within the
gizzard. The gizzard of some species contains small pieces of grit or stone swallowed by the bird to aid
in the grinding process of digestion, serving
the function of mammalian or reptilian teeth. The use of gizzard stones is a similarity between birds and
dinosaurs, which left gizzard stones called gastroliths as trace fossils.
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