Our chickens are dying and we can't figure out the cause. Most of them will act sick, with the back tail raised and they will just sit and not eat or drink then die a day or two later. I have de-worming medicine in mixed in their food, we sprayed the coop down with bug spray, we do not see any signs of bugs and they are still dying. What could be the cause of this, and what should we do? I added tetracycline hydrochloride powder to their drink, but they don't like drinking that.