I feed my chickens both oyster shell and roasted and crushed eggshells for calcium. They never finish the oyster shell. When I put out fresh, they are not to interested. When I feed at night, I put out eggshells with the oystershells and they go crazy for it - they leave their feed to eat the eggshells. I am mixing my own feed now, so I worry they are short on calcium to crave the eggshells so much. I don't have many soft eggs, but think the shells are a bit weak compared to those of commercial eggs. Any idea why they wouldn't like their oyster shells or why they like the eggshells so much better?