Oh wow, after Google Image searching, I think you're totally right. It is near the base of her tail. I can't believe I've never seen or known about that before!
Well, hopefully someone else finds this post useful in the future and doesn't freak out like I did.
Today I was sitting with my chickens and one of them ruffled her feathers. When they do this I'll peak in to check if anything is going on with their skin, and sure enough I saw what looked like a larvae attached to her back. Upon further inspection, I tugged on it and it seems like a hard...