The nub that's at the base of their tail is their preening gland! The chickens stimulate it and it releases oils needed for keeping their feathers healthy! Some chickens stimulate their gland too much and it can get infected.
This post here has some good information on feathers! https://www.backyardchickens.com/t/320003/how-long-for-feathers-to-grow-back
Thanks! Glad we never touched the preening gland and it doesn't appear infected. Read article ... said it could take 90 days. Not sure what to do but definitely not impressed with aprons. We really appreciate your help.
Mattie (the feather picker) sleeps on the roost on one side by herself and the other chickens are all on the other side huddled up together. Thinking she might need to go.