We have lost three of our 28 x-battery hens in the last month and now one of the youngest is shaking her head, yawning and scratching like a dog. All the hens seemed healthy, nice red combs, eating and drinking normally, fully feathered and bright eyed. One looked a bit off just at the end of the night before she died. Didn't roost and basically looked a bit tired and cold. I put her in a nest box none of them use and put loads of fresh bedding all tucked around her and in the morning she had died. I can't see any parasites and checked the head shaker out pretty thoroughly tonight so I'm at a loss. Thinking maybe parasites I'm not seeing or a contagious respiratory issue. Would my best bet be to dust for lice and mites just in case and throw some antibiotics in the water? The coop is not very well ventilated right now either as I closed the windows one cold morning that they were all piled together. Maybe I should open them up? Cold better than moist? Any advice would be great!