My flock outside (9 hens) had some sort of URI, I had them tested for everything at the poultry lab and they came back negative - even no CRD. I did put them on Tylan for 10 days and that helped some. At the time I had a batch of chicks in the brooder, but even with the best biosecurity practices all the chicks are now sneezing.
The only thing I can think to do is give them Tylan and see if it helps. I also have Baytril but would only use that if I got desperate. My girls have never gotten sick enough to stop eating, they just sneeze and cough time to time.