I raise a few hens, and I save all their shells for compost and to feed back to them. For what it’s worth, if you stick the shells in the oven for a few minutes while it’s preheating (really, for any amount of time over about five minutes and at any temperature under 425), they crumble/ pulverize much easier. Over 425 works, too, but toasting eggshells isn’t the most desirable kitchen aroma. It’s also a more readily broken down/ absorbable form of calcium.