Problem with only eggshell as a calcium source is there's a declining amount of calcium available in them, if no additional calcium is brought into their diet (whether through feed, oyster shell, limestone, etc). I now mix eggshell with the oyster shell to stretch out my dollar, since real oyster shells aren't cheap, and since that helps encourage birds to take in calcium as not all of them like the oyster shell.I've never bought the oyster shells because my parents' chickens never cared for them so I figured the eggshells was a good source of calcium.