I clearly feed my chickens too well. I discovered that I had about 50 mice living in the coop last winter, when I had young chicks in a dog kennel inside the coop. They nested and multiplied underneath the kennel without me knowing it. I don't think a single one was eaten by chickens! I wasn't too worried about diseases, these were healthy, chubby mice with shiny coats who grew up on medicated starter feed...
I have a small, young, flock and it seems as though not all chicken behavior is instinctual. I think chicks that come from an incubaotor and are not raised by their mother maybe don't learn how to free range. The low-man on the totem pole in my flock is a buff orpington who probably was from a hatchery but spent time in a large free-ranging flock before I bought her. She's the one who got the mouse, and hunts down the Japanese beetles. In spite of her low status, the others seem to follow her lead in these instances. I tried to put some hatching eggs under her when she went broody, but the only one that hatched under her was immediately pecked in the head, maybe by her. I don't know. I might try again next summer. I'd like to observe some chicks being raised by a natural mother to see if there is a difference. I would be interested to know of others' experiences with incubator vs. natural raised chicks to find out how much behavior is learned vs. hard-wired.