I have 5 chickens all laying one egg a day each. All the eggs have hard shells and are fine in every way. However, sometimes late in day (just before they roost) I get a single soft shelled egg as well. Sometimes I don't see until the morning when I let them out. Normally they all lay in the laying boxes attached to their house, but I find the soft egg anywhere (under the roost, in the pen etc) Can anyone advise me?