I have 3 Buff Orpington hens. They are 1 &1/2 years old. They have eaten the same chicken food all their life except a month I switched to an organic feed. They are free ranging chickens and they go all over the pastures. They eat Nature Wise laying pellets. They have continuous access to oyster shell scratch. They have always had VERY hard shells. Now their shells can not bear their weight when they go in to lay an egg. There are no chemicals on my farm. I garden organically. I did till in composted pony manure in the garden and they go in there whenever they want. Anyone got any ideas?