I have a couple hens that are acting weird. They climb up on top of another hen's back and it looks like they are mating? They bite down on the top of their head and hold them down while they do this. I now have several hens that have almost no feathers left on their head or on their backs at the base of their tails. I have seen 2 different hens do this. They don't have spurs, don't crow, and look just like the rest of the flock. Anyone have any advice what is going on here?