I just never thought about the chances of it possibly being airborne before. Wow...tunnel vision I guess. Since I am a firm believer in using the medicated feed for babies, I don't see it in my hatched chicks. As you said though, sometimes there will always be some who need more and maybe a few, who will need a prayer in the end. As I said in an earlier post to Casportpony, with something underlying in your flock, it can and usually will become an issue due to the overwhelmed immune system they already have.
So, to take this just one more small step...are they truly building immunities while they are in the brooder, without being exposed to the ground? Would it be enough of an exposure, if this is happening, for them to survive full ground exposure, or would they still need the additional 'help' from the amprolium in the feed?
You've given me a lot to think about and I thank you for that. Going on 3 years with poultry now and I never thought of cocci this way.