My chicks pulled out the tail feathers of one chick and pecked a wound into the tale base until it bleeds. It happened over just one day. The chicks are about 3 weeks old. I took that chicks out, and now they are doing it to another one. I took that out, and they have now started on a new victim. They are being brooded by a hen, and they are free range in the day and in the coop at night. There are 12 of them. They are selecting only one chick to do this to, and when that chick is removed they choose another one. I have never had this problem before. What is going on?