Birds don't have teeth to chew their food, they have a muscular organ in their throat, the Gizzard, that they fill with rocks to grind their food for digestion. You have to feed these rocks to chickens in the poultry "industry". If you go to a farm type store (Farm & Fleet, Tyson's, etc.) you will find large bags of 3 or 4 different sizes of rocks (crushed grannite) and crushed oyster shells for this purpose. Most folk use the Medium or Small sized crushed grannite. The oyster shells usually don't work well as they are light and float away in the rain (besides not being prototypical color and shape).