I started out with 5 sexed chicks several years ago...all raised without a roo in the picture. Two years ago I introduced 4 pullets to the flock that were part of a straight run that also grew up with roos. Last year I placed some fertile eggs beneath a broody hen and hatched 2 female and 2 male chicks. I kept the chicks separate from the flock for about 5 months until they were big enough to protect themselves, and then introduced them. Now the interesting part: Early on, the mature flock of hens would beat on the cockerels, more than the 2 young pullets. But gradually, the cockerels would become more bold and begin to mount the hens. Their 2 brood mates were receptive from the start, but the mature hens were not, and would try to attack the cockerels as they did their business. After a while the cockerels dominated and things were fine. Then one day I found one of the original 5 cowering in a nest box. she wouldn't come out for several days...in the end they had ripped all the skin from the back of her skull and neck...I had to euthanize her. Now within the past week another one of the original 5 is getting the same treatment. I noticed that she won't let the roos mount her. But she is completely ostracized from the flock by the roos. Not much blood, but no feathers and I'm applying anti-pick every 2 days.I don't know how much more of this she can take...I'm going to try and re-home her. It's so strange, the minute the roos get a glimpse of her they go straight into vicious attack mode. Even if I'm right next to her and give them a good kick..they come right back in for the kill. My theory is that the original 5 were raised to maturity and beyond without any roos, and as a consequence, perhaps these 2 were not receptive to the roos, and because they resist, the roos want to cull them from the flock. None of the others appear to resist the roos. I am keeping an eye on another of the original 5 that is spending way more time up on a high roost...I wonder if she is next? All of the flock are healthy. What do you think?