As soon as they have fully feathered at around six weeks.I live on the coast. Weather ranges from 55 to 70 during the day depending on fog, and in the 50's at night. I have 4 week old chicks. They are starting to get the combs on their heads. They get irritable with each other, and one is constantly trying to fly out. We started putting them in their pen a few hours a day and that has helped them be less argumentative with each other. When can I put them outside overnight without a heat lamp? We are quite smitten with these chicks and I don't want to go out some morning and find frozen babies.