Last year I put my chicks out at 5.5 weeks. I put a wireless thermometer in the coop and I added a heat lamp. I kept watching the temp dip down in the coop and as it dropped I kept getting out of bed to run out and check on them. It was down to 20 in the coop, but they were snuggled in a pile near the pop door, totally away from the heat I had so thoughtfully provided. They were fine - I was freezing. The next night it was the same story - cuddled in a pile of feathers and several feet away from the heat lamp, but I only checked on them once. In the morning the heat lamp came out. They didn't need it - it was out there for my peace of mind, not their comfort, and there was no point in risking a coop fire or paying extra for the heat if they weren't using it. That night it snowed.....and we got our last snowfall on June 6th. They thrived. They are still thriving - they are sleek, healthy, and strong.
This year I brooded the new chicks outside in the run. Yep, the run. No heat lamp, just a heating pad. Nightime temps dipped into the upper teens, low 20s every night. These are the strongest chicks ever! Eleven of them are 5.5 weeks old and 4 of them are 4.5 weeks old. They are mingling with the Bigs, going outside for much of the day, and have never had a heat lamp on them - just a cave with a heating pad over it. These little critters are a lot stronger than we give them credit for. I have a new batch of chicks coming today, and they are going straight out to the coop as well. They'll have the same setup - just a cave with a heating pad, and they'll do just as well as this batch has done.
So at 8-9 weeks old, your chicks should be more than ready to start living like chickens outside, even without supplemental heat. Relax - they'll be fine. They might be mad at you for a day or two but they'll get over it.
