JackE I agree 40 is warm, most of the fowl should do fine at least until 30 degrees, when you get a wind chill in the teens or below I feel for them especially my old girls and you can see that they appreciate the warmth when it gets too cold.
Just for the post:
The little heat from a 100 watt keeps eggs coming too. But if left on too long does stress the birds very much, starting in the fall I have the timer set from 5am to 9:30pm so the regular light during the summer is consistent year round
My turkeys don't have any problem at all with any tempture or elements, but they do have the choice to get out of the elements.