They don't seem to be stressed. Sometimes it's supposed to be caused by too much white light, but I think sometimes they just start doing this because something looks like it needs picking. Put the pine tar on with a stick, and the picking will stop. You can find this at farm stores and tack shops. They can't stand the taste.
Cannibalism is a vice developed in confinement.
The biggest cause of cannibalism is confinement. Once it starts, you must stop it immediately, or start losing birds. Haven't had this problem with my chickens. Might want to give it a shot. Worked with the Pheasants.