agreed, the heaters don't keep it that hot, just keeps temps in the 50's (unless I am hanging in there with them for a while then I turn it up a bit) They handle 50's just fine and yes will be fine in 40's ( love those poikilotherms)but that's as cold as I'd like them to be. (same goes for the Parakeets and Silkie chickens)x2
If you are heating the coop and the birds are used to warm temps all winter - they will really suffer when the April snowstorm comes through and knocks out power for a couple of days. Let the birds acclimate to the cold winter temps and work to keep the water from freezing. Cold temps won't kill them if they are used to it.