I adopted 7 chickens and a rooster about 1 year ago. One of my chickens hatched a chick which we rescued and raised in the house (under proper temps etc.). The chick is now 2 months old and we don't know if it is a boy or a girl yet. We have notice that there is this "nub" on on it's back at the base of the spine right where the tail feathers begin. The chicken/rooster will fiddle with it with its' beak and rub its' head over it while it is preening itself. What is this thing?