I have 3 hens (one silky, one Barred Rock and one Dominique) all of whom have suddenly stopped laying eggs. They are three years old and had been laying eggs regularly until last week. They are not hiding their eggs. There are no egg stealers. They do not even go into the nest box anymore. I do not think they have been stressed. It has not been unusually hot. They are not molting. They do not have mites or lice. They otherwise seem normal. Any ideas?