13 of my 14 hen flock died this past Saturday night and I am at a loss as to the cause. Here are the facts: Hens consisted of 4 Black Australorps, 5 Buff Orps, 3 Red Stars, 1 White Orp - ages ranged from 7 months to 2 years. The only survivor was a 4 year old RIR. The birds expired in their coop Saturday evening between 8:00 pm and 7:00 am Sunday morn. It it appeared as if they died in their sleep. No evidence of predation (run and coop are very secure). No evidence of disease at all. These birds were thriving by all accounts (eating, drinking, bright red combs, active, laying, solid poop, etc.) Their coop is 16 feet long and 4 feet wide with 20 feet of roosting area and 8 nest boxes. The coop backs to a run that measures approx 15x30 feet, however the birds are free ranged daily on an acre of land. The waterer is in their run and is clean with water changed out every other day. The feeders are also in the run and feed is clean and dry. The bedding in the coop and nest boxes is pine shavings and was completely changed out about a week and a half earlier. As the birds are free ranged, the bedding stays clean for a while. Every two weeks or so the coop floor and boxes are spot cleaned to remove manure. Given the recent cold temps I installed a heat lamp in the coop which is on a timer to go on after dark and off at dawn. For about 3 weeks this worked fine. This weekend we had a thaw with temps getting near 40 degrees. Saturday night I left the rear door of the coop open. The door is a flap design that measures the width of the coop and extends from the floor to approx 20 inches above the floor. My suspicion is that the air inside the coop became unbreathable. Was it too hot? Seemed unlikely as 40 degrees is still pretty cool and the back door was open. Ammonia? Also seemed unlikley as the bedding was recently changed. Some other gas? I can't help but think it was my own actions that caused this, I just can't figure exactly what. Any input would be appreciated.