I'm sorry about your duck! I don't know about the joint being warm, and what that indicates, however I have a very large Pekin who has twice developed a limp for an unknown reason. I was also very concerned, but since I couldn't see anything really wrong with the leg, chose not to visit the vet and just to restrict her walking as much as possible. The first time it took about two weeks for her to get back to normal, but she did. Her latest limp developed about 4 weeks ago, and was severe at first. I kept her contained in the run until she started getting around a little bit better (a few days), and now she still limps a tiny bit, but is much better. I let her out in the yard with the other ducks for a short time, but I carry her back and forth to the yard and try to just baby her a bit until it's fully healed.