depend on the age and temperature where you are located. I live in Florida and have 6 week olds in the grow out coop without a light. The nighttime temps here have been in the 80's so its been OK It would also depend on the birds being fully feathered. We recently purchased 4 week olds from a breeder the first thing she said was they are large birds they look older they should be kept inside until they are 5-6 weeks old.
I live in Southeast Alabama and our temps. are in the 90's right now and in the 80's most of the night. It's 12:30 a.m. here and my computer says it's 79 degrees outside.
With our first flock of chickens, we were very careful and waited until they were almost fully feathered to let them out with no light. Now that it's so hot and we have raised a lot more chicks, we let our newest ones out in a pen (with no light) at about 5 weeks old. They seemed to be getting TOO hot in the brooder. They have been in the pen for a week and are doing very well in there so far.
It probably at least partially depends on how hot it is where you live.
mom2chicksandpups and micki I am so jealous! 80's and 90's ! It was 40 degrees last night here in NH. Summer could show up any day now...any day now...please? We had a frost on June 1st. It didn't do my garden any favors.