Hi I had this happen to my last flock.sadly I lost all of them. It seemed none had symptoms but when I was down to my last bird I heard a slight wheeze. So in my case it was a dreadful respitory illness. You need to look listen and feel for ANY symptoms you can find. Any mucus or sneezing or coughing or any slight wheeze.? How is their poo? Color and consistancy? Do they appear puffed up or sleepy? Does their tail bob when breathing? Bobbing tail could signify a breathing struggle. Any unusual behavior before hand? Any limping or paralysis. I know this is a lot of questions but each question could be a clue to a particular illness. When we're they wormed last? Are they thin? Do they have any external parasites such as mites or lice? Does the coop have any mites or lice. Is their any access to wet feed? Moldy hay or wet bedding? All these things can lead to clues. We would need some clues to help figure this out so please check these things then answer back. What ever it is it's likely they all have contact with it weather it be something in the food or bedding or a virus. If you can give us some clues by answering these questions we might be able to figure out what's happening. Best wishes also is their water clean or is their algae growing in it? Certain algae can also be deadly to chickens.
