Went to the coop at 1pm today and found 4 of our 17 chickens dead, there isnt any signs of a predator getting in , no blood or feathers torn . the birds are still all together. One hen was out in the scracthing area hunched/crouched over with her eyes shut just slightly breathing.The other 3 were on the ground dead. When we checked on the birds last night at 8pm they were all in the coop doing fine. What could be the cause of this? the rest of the flock is fine/normal. Please any suggestions welcome