Many birds seek out and swallow small particles of stone to aid digestion. The grit remains in the gizzard. This is a big muscular pouch just above the intestine. The gizzard grinds the grit together with whatever else is consumed. The process is similar to a human chewing but, of course, a hen has no teeth. This gives rise to the phrase "as rare as hen's teeth". Commercially produced feeds are prepared in wasy which mean the presence of grit is not essential to aid digestion. Indeed battery caged hens have no access to a natural supply of grit. But if your chickens are free range and eating a mix of foodstuffs then grit is important. You only need to provde grit if your birds are kept inside or if the ground is frozen or snow covered for days. Even then it is only vital if their main feed is whole rather than processed grain. Grit is sold by feed suppliers but a handful of river bed materials will do just as well. Beware grit from landscaped areas such as parks or roadsides as this may have been treated with chemicals or exposed to other contaminants.
