Just thought I would mention that you are right about penicillin, it does work for a lot of infections such as upper respiratory ear etc., but it really isn't considered as much of a broad spectrum antibiotic. There are other antibiotics for specific things such as sulfa antibiotics for urinary problems like Bactrim and Keflex for wound infections along with many others. But as you mentioned, I notice that penicillin seems to be the drug of choice for a lot of chicken illnesses and I wonder if that is because their usual bacterial infections are upper respiratory.
I think it's the drug of choice for backyard chicken keepers because it's what's at all feed stores, not because it's a good one.
-Kathy