When I was speaking with a Michigan State University vet, he told me that a lot of chickens will get "sinus infections" during weather changes...especially like we've been having. Generally, the only symptoms will be sneezing, rattling, and sometimes coughing. Otherwise, the chickens act normal...but they will have no nasal discharge or eye problems. He said he always just recommends any of the tetracyline antibiotics...like terramycin. I've been doing a lot of research on illnesses and diseases, and this vet has taught me A LOT. Biosecurity is the best policy, but chickens can still catch a bacterial infection from seasonal changes.