If overdosing chickens is such a likely problem then why do people not have issues with all their chickens kicking off from pigging down calcium? I believe it is because they don't eat it if they don't need it, I personally have never seen a rooster eat my shells and the slow rate they are consumed tells me none of them eat very many anyways. I think a chicken is more likely to overdose on calcium laced feed as it had no choice but to eat the calcium if it wants food