One of our hens, I'm not sure which, laid a half-size egg without a yolk. The hens are from 11-18 months old. One, a Jersey Giant, doesn't lay regularly in the winter. Is this just her system starting up as the days lengthen?? Could it have been laid by one hen who is the far and away favorite of the two roosters (read: bred frequently and starting to look shabby - we moved her to a friend's house today)? Thanks!