Hi I am currently feeding purina flock raiser to my ducklings and chicks. They will be moving into the 1 year old chickens and duck pen that are eating layena. I plan to feed them all flockraiser when I combine them. I know the laying hens/duck will need calcium so i should give free choice oyster shells. My questions are..Can I get Oyster shell at Tractor supply, Will the chickens/duck know to eat it, and How should I give it to them(in a dish, etc.). Thanks!