Sulfa is good for lots of things. Antibiotics can help if it's bacterial, but if it's viral they won't. Sometimes it's hard to know if it's viral unless a necropsy is done. I would only treat the bird, or birds that are showing symptoms. It's best to give it orally direct to the bird so that you know they got the correct dose, if they are ill they often don't drink enough to get the correct dose of medications mixed in the water, and some medications won't mix, they will just settle out. Enrofloxcin that you can get without a prescription is usually liquid, Amoxicillin is usually capsule or tablet.