We've always fed the egg shells back to the chickens. Generally speaking chickens won't eat their own eggs unless they are already broken (and the eggs generallly only break if they are thin shelled). It is natural for them to do so for two reasons.....1 they are oportunistic feeders - they eat what they find.... and 2 to clean up the shells when their chicks hatch so they don't attract predators. I have watched hens do this after their eggs have hatched! The first clutch I had I went out to check the hen and found several chicks and no shells, the next time I got to watch and sure eneough mama hen ate the shell!