Can you post a pic of it? Is there any chance she's broken her wing?

Unfortunately, growths like this are not good news. If it is hard, it is likely tissue and not fluid, so is not a cyst or abcess (which is more treatable). I'd take her to the vet right away for a diagnosis.

My vet has told me in the past that chickens are susceptible to something like 300+ types of cancers and growths. I once had a Polish hen who had a growth like the one you describe at the base of her tail--otherwise she was healthy and perky. All we could do it keep her comfortable and happy until she went, about a week later. But she was active and in no pain right up until she died. The growth just sucked all of her body's energy and nutrients away.

I wish I had happier things to tell you, but that was the experience I had with my hen and that's the info I got from my vet, for what it's worth. I'd take her to a vet and get a diagnosis to start, meanwhile keep her warm, comfortable and quiet, and make sure the other birds don't peck at her. Make sure she is eating and drinking normally, and that she doesn't have any other symptoms.

Good luck with her!

Velvet ~:>

Hey caliopi, how is the hen? Any pictures of the lump?

Velvet ~:>