Grit is made primarily from ground up minerals and sand. Some of the substances used are insoluble and cannot be digested. These include silicates and sandstone. Soluble, and therefore digestible, grit is primarily made of limestone, which is calcium carbonate. The soluble grit can be made from cuttlebones or from ground up oyster shells. Grit for birds is specially formulated and is not the same thing as offering sand from a sandbox or small pieces of gravel. The purpose of grit is thought to help the bird grind up difficult to digest items. By grinding, it is thought that these food items become better utilized by the bird. The grit is mostly undigestible material. It will stay in the ventriculus, or gizzard, for months to years until it is passed in the stool. Since the primary purpose of grit is to help remove husks and shells from seeds, birds that ingest whole seeds, like doves, would likely be the best candidates for needing grit in their diets. In the psittacine world, most parrots are able to remove the husks and shells with their beaks. For this reason, these species do not require grit in their diets. In the passerine world, canaries, finches, etc, are also usually able to remove the shells with their beaks and do not typically need grit to help in digestion. Birds on pelleted diets do not need grit. Whether or not you offer your bird grit is a personal choice. Many birds have lived long healthy lives without ever ingesting grit. When fed an easily digested, appropriate diet, grit is not a dietary requirement.