If feeding eggshells, it’s best to cook them in the oven a bit and then grinding them with a mortor. 250f for 10 mins should do it.

Oyster shells work well if you can get em. Other bivalves can work too but oyster is easier for the chickens to crack apart. You just want to bake it at like 250F for 10 mins

Here’s a few other methods. https://www.backyardchickens.com/articles/feeding-your-chickens-oyster-shells.77372/