I know everyone says to offer oyster shells to your chickens to make sure they get enough calcium (and avoid soft-shell eggs), but my hens will not touch it. I think it might be because the pieces are too big, so I tried to crush them, but they are pretty indestructible, so that's not working. I bought them at TSC and don't see any finer option there. They are getting free-choice organic feed, and I have been doing the bake and crush egg shells to feed back to them, as well as plain Greek yogurt. Then I thought I would augment the yogurt with powdered calcium carbonate, but now I'm not sure that's the right form of calcium. Is calcium citrate better? I read one person's mention of crushing cuttle bone (for birds) because it is very high in calcium citrate. Any advice?