After a couple of months of trying to find out what's wrong with my chickens, it looks like I might have an answer. My vet (who takes care of birds but knows nothing of the epidemiology) finally got hold of the state vet. She was going to try to send off samples so we could finally get an answer. After talking to the state guy, he said that Coryza was almost unheard of here, and that it was extremely likely that it's MG. He said that they've been seeing a lot of it around the state. He prescribed an extended course of Tylan- 2 months on, and another month AFTER the symptoms disappear. So i've been doing the right thing, I just didn't so it for long enough. He also advised vaccinating any new birds, and NOT culling the flock. It apparently wouldn't do any good, since it's SO prevalent, and any replacement birds would probably come down with it as well. So yeah, MG is pretty much everywhere. Vaccinate! Anyone in the area want to go in on a bottle of vaccine? It's a thousand doses. lol.