I'm sorry, but what I'm trying to explain is that they are inside a building for well over 12 hours a night without adequate ventilation. It only takes an hour or two to build up bad air.
All my buildings have huge windows open all year except one.
That building has a fan blowing cold air into the building around the clock expelling fungus, virus, bacteria and dust.
I've never had a respiratory problem in birds in my life.
They are outdoor animals with tiny respiratory systems that can quickly be compromised. Chickens can live in trees in temps well below freezing.
You have a bird with a respiratory problem that hasn't been diagnosed. I'm just trying to give you possible causes. There is no treatment other than what you have already done without a diagnosis.