Weird growth near tail feather

[Hereislyss]

Found this weird thing on my buff Orpington on the tail feathers as I was checking to see if she was being bothered by any mites, etc. there are feathers growing out of it - but seemed so thick for new feather growth? Never seen anything like this before, anyone have ideas?

Sorry, it’s night time and was hard to get a good photo.

 

[BlueBaby]

I believe that is a gland that they use to preen themselves, and I think it's some type of oil that they store for it.