One thing I would do is to pick up each of my chickens and listen closely to their breathing, listen for congestion and look for any possibility of mucous around their eyes or nostrils. Upper respiratory diseases are common this time of year and they can be subtle. Chickens are so stoic, somtimes a URI can go undetected until the bird drops dead. Are any of them looking hunched over? What does their poo look like? Look around for bloody or weird looking stool, take pictures if you find any. But also, check those lightbulbs. I think that there is a class action suit against GE.
