This is an interesting situation. Here is my experience. 3 years ago, I had some sort of respiratory illness in my flock. None acted sick. All continued to eat and act energetically. However, much coughing in several of my birds. Took one to the vet who gave me very little hope. However, I went to my local hatchery and they told me not to worry and to give my flock a round of Durymycin. I did not want to use antibiotics but I did not want to lose my flock either, so I did give the antibiotic. All survived and were better in a day. I continued the treatment for 5 days. Two golden laced wyandotte hens continued to occassionally make coughing noises intermittently. Now, these two same birds are coughing when they eat. Still looking great, acting well, but coughing. I am watching my other birds, waiting to see if any others start coughing. If others begin, I will treat my flock. I wonder if anyone else has had this experience? Can birds get colds, allergies or have chronic cough from a previous infection? The vet did not find Marek's or any of the big bad chicken infections when she treated my bird, so I do not think I have some type of horrible infection in the flock but I would be interested in hearing opinions.