We did not even use a heat lamp until they went outside in their coop at 5 weeks old... Since the day I brought them home i used light bulbs... I think we started with a total of 140 watts... (1- 100w and 1- 40w bulb), and dropped down to two 60watts for 120w, the next week,, and kept dropping until we reached 70 degrees,, I think it took about 5 or 6 weeks. Then they went out into a very good size coop,, Thats we when we added a heat lamp at night for about a week..now they have no lights and they seem to have adjusted very well...
Just remember, God took care of them "all natural" long ago.... So I am sure your babies are going to make out just fine!
Good Luck and have fun..