Purina, 5 lb. oyster shell supplement, helps support healthy and sturdy egg shells. The purpose this supplement serves is to provide calcium and grit. Hens that are laying eggs need a source of calcium to replace what they use in making their egg shells. Oyster shell"s function as grit to grind the stuff these chickens will also eat. She will digest and deposit oyster shell calcium, reconditioned as egg shell and in their poop which I can add to our garden soil, providing a more bio-available form of calcium for our garden greens to absorb.