What would you treat with antibiotic? The only thing I use one for, and I do mean the ONLY things, are internal reproductive infections or bad wounds. Those need something like injectible penicillin, not weak Terramycin. I choose to never, ever treat respiratory and you don't want to get into that habit. A healthy chicken with a strong immune system usually doesn't need it. Since most diseases are herpes types (many of them stay with the bird, even when it appears to recover, and make it a carrier), I choose to euthanize if any of mine ever contract respiratory illness. That is my own choice and I've been hammered for it over and over, but if you start treating respiratory illness, you'll have to do it over and over and over throughout the life of your flock. Not what I want for mine, but it's your choice.