Devastating Unexplained Overnight Loss - Sudden Deaths -

[suburbanminifarm]

I am so, so sorry--It does'nt sound like you did anything wrong!

Could they have gotten in to rat pioson? Antifreeze? Have you asked your neighbors if they laid any down? Have you examined the bodies very very closely and is there any red spots or bruising on the skin? Check in their beaks?

 

[Mac in Wisco]

It does sound like heat prostration. How warm does it get in there with the heat lamp on? Is the coop well insulated? Is there any other ventilation besides the hen door that you had open?

Regardless of the 40 deg outside temperature, it is possible that they died from overheating, depending upon how well your coop is insulated and how well sealed it is. The door down low may not have let out enough heat. Especially with the long coop that you describe.