The head is a popular place for chickens (hens and roosters) to peck. Do you know if those hens are low on the pecking order? I have heard of many hens having that problem and the owners didn't even have a rooster. I have 2 roosters (their so nice, but I dont know because..) that make the hens lose back feathers. If it is the rooster, before you get mad at him, please remeber that he is just trying to do his rooster dutys. Since he seems to like 4 other specific hens those girls might be broody. My roosters love my BA's ( they are broody) They love 1 of my BO's (she is broody).
Good Luck!
Good Luck!