Chickens really do have a pecking order – they literally peck at each other to establish who is the ‘top chicken’. If they peck another chicken to the point where it bleeds, they continue to peck at the bleeding chicken and they actually can kill it. The upside of this is that chickens can be not all that smart, and there is a way to take advantage of this.
Chicken keepers have figured out a way to add a new chicken to the flock safely, without triggering a massive pecking on it. They sneak the new chicken into the flock at night, right onto the roost with the other sleeping chickens. When they awaken the next day, they accept that new chicken as one who has been there with there with them all along. . .after all, the new chicken was there on the roost when they woke up; she must have been there when they retired the evening before.