I know for a fact that they will,pardon the term, recycle their old poop to extract any calcium that might be in it. I have seen my girls eat it even though they are on layer with free choice oyster/eggshell at the ready, right from the coop floor. After its been composting it could pose a problem but not sure. When they free range they are constantly digging in yard waste around the yard and who knows what is in that.
