Please could anyone help. My chickens are dying at a alarming rate. Chickens are various ages. They have roam of 6 acres and large coop at night, access to fresh water and food at all times. When they become ill I will find them hunched and fluffy. Within 3 days they are dead. They look fine up to that point, nice feathers, happily eating and acting normal. We have now lost 12 birds over a 6 week period. What could this be? Please help!