The symptoms you've described in your birds, their ages, the fact that they've recently been put out in the coop....all are classic for coccidiosis. Have you not tried treating for that? If that's what your birds have antibiotic's will not cure it. Some may be able to survive it, many will die, as you've seen already. At their age, unless you are seeing other obvious signs of disease such as respiratory symptoms I'd highly suspect coccidiosis is what you are dealing with. It would certainly be worth it to treat for it, you just might save the rest of them. If that's not what it is then at least you've ruled it out and can move on. You can also get a dead bird necropsied, preferable by a state lab. Then you will have an exact diagnosis as to what is going on in your flock. But in my humble opinion coccidiosis is your biggest suspect.
