We had some of the same sorts of problems, we lost three birds in a matter of weeks. The symptoms were dirty hind quarters, followed by them looking very tired and not acting themselves. We looked into it by bringing them into a local vet and brainstormed that it might be some sort of upper...
Wow, I'm in the same situation you're in. I live in West Bloomfield, MI, and we've recently gotten our poultry from this nice little house/farm in Brighton also. We have neighbors on either side of us on our one acre strip of land. So far no problems have arisen from the county.