They probably don't need more. Mine go nuts over the eggs shells, and they are gone in a flash...and they are left with nothing BUT the oyster shell if they need the calcium.Interesting. All mine snack on the crushed eggshells in the container but ignore the oyster shell.
Onc e a week I bake the shells in the oven, crush them, and put them out. If I put them out in the morning, they are always gone within an hour or two! It's kinda like asking a kid 'do you want cake, or broccoli" they scream 'cake, cake, cake,.'..
But when the cake is all gone, and they are hungry, they'll eat the broccoli. (well, at least I would - but then as a kid I ate everything but watery soup and hot dogs! I even liked liver and onions as a kid (as long as it wasn't overcooked and like shoe leather!))