Mini egg

We have a couple of chickens who seem a bit stunted. They were perhaps ones we had to help out of their shells - or maybe the rooster has some kind of genetic mutation going on (they come from unrelated mothers so assuming it would have to be the rooster).

It isn't very clear in the picture, but the one I think laid the mini egg is the black one in the middle. She is smaller than the others by quite a lot though it doesn't look like it here...