Q: Do I need to give my hens oyster shell or supplemental calcium?
A: If your hens have a good, "complete" layer feed, they may not ever need oyster shells or any supplemental calcium! However, it never hurts to offer it free choice (and it is inexpensive and lasts a long time). The reason is that some flocks that spend most of the day foraging, or are given too many treats, in which case they don't get all the calcium they need from their feed. This can cause weak or irregularly shaped eggs, slow laying and can even cause behavior problems like egg-eating, because they're so desperate for the calcium they need! (We recommend you cut down on the treats, too!)
We do not recommend oyster shell for pullets that haven't started laying yet, however.
Some people prefer to save their egg shells and offer those back to the hens--that works, too! However, please remember that a hens' own shells will not provide her enough calcium if she doesn't have another supplement, since she also needs calcium for other body processes. If you choose to offer eggshells back to your hens, be sure to bake them first (350* for 10 minutes or so) to kill any bacteria on the shells, and make sure they are crushed finely enough that they don't look like eggs... or else you may be teaching your hens to eat their own fresh eggs. What a headache!