I have had a few chickens die over the last week or two. One will start looking lethargic, just laying around or standing and sleeping sort of. Within the day it will be dead. I've had 4 die from it, whatever it is. They just start looking sleepy. I'll remove them to another area, and after a bit they die. No other symptoms that I can tell. I have 60 chickens that I adore (and have a small fortune invested in). Each of the dead came from different places. Our local feed store, the tractor supply, and two private breeders. They have been in a fenced in yard that I put them in at 1 month old, until they are ready to free range with the adults w/o getting roughed up. Any ideas?