Last year mine were out with no supplemental heat at 5.5 weeks in Northern Wyoming, and it was in the 20s in the coop at night. Oh, I felt guilty and put a heat lamp out there for them at first, but they didn't even go near it...they piled in a heap of feathers and feet in the corner by the pop door. I kept getting up to check them the first night. The second night I only checked them once....same story as the night before. The third day the heat lamp came out. If they didn't want it I wasn't risking it. That night it snowed. And it snowed off and on until the 6th of June. If I'd have waited for the weather to be "warm enough" they'd have darn near been old enough to start laying eggs in the brooder.
Now I start them outside in a pen in the run from the beginning. No heat lamp - just a heating pad cave covered with straw. My third batch being done this way is out there now, and it's been down to 19 degrees. They are doing just great! All healthy, feathering well, and loving the entire set up. They spend most of their time running around and exploring, and if they feel the need for a little heat they run into their cave. But as soon as they are warmed back up, they scurry back out. Two nights ago we had a power outage sometime during the night. Winds were howling, sustained at 30-40 with gusts of 50-60 and the snow was coming down sideways. The power coming back on about 4 am is what woke Hubby up, and he woke me up with two words, "The chicks!" I had a batch of week old Brahmas in the pen. We hightailed it out there. The heating pad was off, and we don't know how long the power had been off. The straw inside, around, and on top of the heating pad cave held enough residual warmth to sustain the chicks and we got that thing turned back on real quick. When I went out later to check them, I was greeted with the sight of 8 little hineys in the air and 8 little heads in the feeder. These little critters are tougher than we give them credit for, I think.
I think you are right! My girls don't even go under warmer very often anymore and by 6 weeks I am removing it all together
