Help!!!

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?

 

The birds are all around 1 month in age. They are in a 8x16 run with wood shavings for bedding and a framed in section of dirt for their dust baths. I gave most of the other info in my post, but I'll repeat. Four have died in the last week or so. Another two are exhibiting symptoms now. They start acting sleepy and either stand or lay down, puffed up with eyes closed. Once they start exhibiting symptoms, they die within a day. None of my adult flock, which runs the yard free range around the smaller run, is sick. The birds that have died came from 4 different places.