Yesterday as I was collecting eggs I had a surprise. It was much later than I normally collect eggs and the birds had gone to the roost. I have a old brood box that I just left in the house so some of the birds roost on top of the box. I happened to look on the box and there was an egg. I got it and much to my surprise, the outer part is like a rubber ball. Does any one have any idea what may have caused this. In my few years of chickens I have never had this happen. And these are young hens, but all have been laying for about 4 months now. The only exception is a VERY old Rhode Island red that has not been laying at all. Also, there is a feeder of oyster-shell in there run and it seems to be getting eaten.