I would still quarantine, just as a precaution. Even though your birds are healthy, and the new bird is technically coming from the same flock, it's still possible that one or more of the farmer's flock could have picked up a disease, or may be harboring an existing disease that was already in his flock. Also, things can change in a month's time. But, you'll have to make your own decisions regarding how much disease risk is acceptable to you. A lot of people bring new birds home and stick them in their coop the same day. It usually works out ok, other times the new birds bring in mites or lice, or a disease that has potential to wipe out the entire flock. Others go through a lengthy quarantine, while others do an ineffective quarantine. (not much space between the new and the old, or the old are free ranging, thus having access to close proximity of the new, flock keepers moving between flocks without sanitizing.) What ever your comfort level, that's what you should do. I don't think there's a one size fit's all answer here, merely expressed opinions about what others would choose to do in a similar circumstance.
