Egg shells too. I'm not sure how they'd be processed, but I'm guessing bake them in the oven and disintegrate it all in a food processor.
I had a discussion with other members about using eggshells as fertilizer in the garden. Turns out that the calcium in eggshells is bound up and the only way to break the bond is using something like a vinegar solution and letting everything soak for a week or more. Crushing up the eggshells will not break the chemical bond and release the calcium to be taken up by the plants. It has to be cooked, crushed, and soaked in a vinegar type solution.

I have a separate calcium feeder for at will feeding for my chickens, but my bag of calcium I bought 4 years ago is still mostly full. My chickens don't eat much supplemental calcium. I suppose you could puree the eggshells in a food processor down to very fine bits and put it into a separate feeder. But that takes time and I'm too busy right now for things I don't really need to do.