Grit (supplement)

Grit is a material eaten by birds to aid in their diets and digestion. Wild birds find grit naturally while foraging, and farmers can purchase grit for their domestic fowl.

Composition
There are two kinds of grit:

Soluble grit dissolves in a bird's digestive system. It is usually made from calcium-high materials; the calcium helps the bird in egg shell production. Soluble grit can be made from limestone or coarsely ground or broken seashells, like oyster or mussel shells; this kind of grit is also called shell grit. Other uses for shell grit include protecting plants from slugs or snails and for aquariums.

Insoluble grit consists mainly of crushed stone (though often with additives). These do not dissolve in the bird's digestive system; instead, they remain in the gizzard for weeks or months to help the bird grind up its food for digestion. Stones used may include quartz, granite, feldspar, and phosphate rock. Insoluble grit is found in the gizzards of birds that eat plant parts and some that eat invertebrates. Because of its grinding function, this kind of grit is sometimes called "hen's teeth."

The size of grit particles varies widely by species, from less than 0.1 millimeter in diameter to, in the case of larger birds like ostriches, over 2.5 cm in diameter. Juvenile birds will often ingest smaller pieces of grit than adults, as in Sarus Cranes. Grit size also varies with birds' diet; larger grit helps birds grind down harder, coarser food more efficiently. The kind of grit used may also change seasonally, whether due to varying availability of grit or varying availability of food to be digested. Sex and reproductive status are usually not associated with grit size. Grit that starts off in rough or angular pieces may become rounded off as it is used in a bird's gizzard.

History
Grit has been known to avian researchers since at least the late 1700s. United States federal and state researchers began dedicating significant attention to grit by the start of the 1900s.

Grit use has been studied in poultry, wild gallinaceous birds, and waterfowl. For poultry, the aim has been to improve the output of poultry and egg farms. For waterfowl and gallinaceous birds, there have been many avenues of research: their status as game birds, their population densities, any potential role of grit in lead poisoning, and pesticides that grit may include.