Eggshells alone isn't the best choice though. The benefit of the oyster shell is that it takes longer to grind down, so it's still getting worked on overnight, while they're forming the eggs.
I hate how wasteful mine are about it. I use a baby pig feeder to keep it contained, but they fling a bunch out to find pieces they like best.
Try putting the OS in something like this dog feeder. I think I got this at a yard sale for a couple of bucks. There's not much waste, and I don't have to fill it very often! The white stuff you see on the floor is the shredded paper we use as floor litter and nesting material, and that's grit, of course, in the skillet.