I use a small pail/dish near the food. Drain holes for any water that might get in.
Their own eggshells alone will not provide them with enough calcium. Oyster shell is still needed. Also it would be highly unlikely that they would put the two and two together that eggshell pieces come from eggs.I have a bucket outside where I keep used eggshells. The eggshells dry out and I crush them up to feed my chickens. It's a great calcium supplement! I do recommend mixing it in with their food because they could develop a taste for it and start eating their own eggs.
I still add oyster shell to their feed, I also feed the eggshells to them separately.
If your concern is about calcium, try purchasing a black soldier fly. Aside from its high protein content, it has high calcium content that helps our chicken to have strong and beautiful egg. I am using it for a while now and it save me a lot from my expenses because I do not need to buy an oystell shell just to provide the calcium needed.
