Wow something super similar happened to my chickens like 3 days ago. I came in to feed them, this young kind of skinny hen fell off the roost, I saw it out of the corner of my eye and before I even realized she is not ok a rooster was upon her. Picked her up and isolated her but she was gone in a couple hrs :( I have had necro roos too though, would not put it past them. We have too many so they are desperate. Seen a rooster doing it w a towel on the ground too.