I keep a container of oyster shell available to my chickens at all times. But once a week or so I grab the older eggs out of the fridge, hard boil them, crush them up shells and all, and feed it to them as a treat. They love it, the extra protein is nice (especially during molt) and they get a little calcium boost. Personally I don't think the egg shells are particularly beneficial as far as calcium goes, but it makes me feel like I'm doing something nice for them so can pat myself on the back. Plus it gets rid of older eggs. I've never had one go from eating those to eating eggs in the nests. I guess that hard cooked and crushed they bear no resemblance to the eggs laid out there.