The gizzard is a very muscular stomach that is found in birds, earthworms, and other animals.

Often, animals that lack teeth will swallow stones to aid in digestion. "A bird swallows small bits of gravel that act as 'teeth' in the gizzard, mechanically breaking down hard food such as seeds and thus helping digestion." (Solomon et. al, 2002) These stones are called gizzard stones and are usually smooth and round from the polishing action in the animal's stomach.

Dinosaurs are also known to have used gizzard stones. When found in association with fossils, they are called gastroliths.

Sources

• Solomon, E.P., Berg L.P., and Martin D.W., 2002. Biology Sixth Edition. Thomson Learning Inc., Australia, Canada, Mexico, Singapore, Spain, United Kingdom, United States pp. 664