OK well then you have a respiratory illness, OR it's the hay. I would use horse shavings (pine only) like you get at
TSC, replace them. Hay is dry, but because of the curing process it harbors mildew spores. It only takes a little moisture added to humidity for the spores to become a problem for the more sensitive respiratory system of chickens (which is why even some horse people wet it down completely, soak it, before feeding if their horses are sensitive).
You will want to use the Duramycin at least - all the birds in that flock, at least 7 days if not 14. Tylan in the water would be an even better choice.
For nutritional support, because you won't be able to add anything to the water, you can spray (with a hand-held misting type spray bottle) "Fortified" wheat germ oil on the feed twice weekly. it's sold for horses and must be "fortified" with A, D, E. A is a super respiratory healing and immune system building vitamin, E is for inflammation (sinuses) and healing. Spray only - do not pour.
If you give Tylan soluble in the water (again HIGHLY recommended because you want to stop this fast) you will be able to give yogurt as a treat in a damp mash once daily to prevent secondary diarrhea and yeast infections from using antibiotics. If you use Durvet, then you'll want to buy Probios from the feed store or
TSC and use that instead in a damp mash daily (never in the water despite the label). By the time you buy two packages of Durvet 10 and probios, Tylan is way more effective and probably less expensive.
Cease the scraps - just feed them their fully fortified complete pellets, grower or layer depending on how red their combs are. You can start giving free choice oyster shell now so that they build up calcium for laying time when you will then give them laying pellets.
Good on the healthy mouths - that eliminates some bad possibilities.
Is there any way you could make a second place for them to stand, not under the coops where the droppings go and the moisture is? That actually might be part of the problem.