Definitions for "Pecking order"
The order of seniority defines who gets to boss who around. It also defines who has first choice at the food dish and who must wait for whom. In ducks and geese this rarely involves violence. It usually is decided by pushing and chasing.
Social rank of each horse in a group; one horse is the boss and the others find their place.
The social hierarchy of chickens; go here for a detailed explanatio n.