Not disputing this, I just want to understand it better. If they're outdoors in the cold all day, then they have some warmth at night, and one night the electric goes out and they don't have warmth, would that really be "likely" to cause fatalities?The problem with heating is that if you run out of heat for any reason example: bulb runs out or electricty goes out, your chickens will not be use to the cold and you will have fatalities most likely.
I recognize that at night they aren't moving around, but on the other hand, they are nestled together for some shared warmth. and even in the daytime when they are outside in the cold, they aren't moving around all the time.
It would seem to me that this potential problem would only be the case if you kept them locked in a heated coop 24x7, but I could be missing something.