We purchased 2 layers from a very clean farm a couple weeks ago that were housed with a couple of roos. They were missing feathers above their tails, like you would see from birds who've been with a roo. We quarantined them, they seemed fine. A week after they were introduced to our 3 hens, our the head chicken (top of pecking order) started missing feathers from above her tail. Plucked clean out of the skin. We don't have any roosters. What's going on? Couldn't find any mites, and they been treated with ivermectin within the past 30 days. Any ideas???