I've looked on the BYC site for a definitive answer as to when chicks can start living outside full-time. Do you think my chicks that are being raised seperetly from the Mama hen/chicks can be cooped up at night outside? I'm thinking about giving them a seperate part of the coop to sleep in at night. One of them is about 2 1/2 weeks old and she's being raised with some older pullets that are about 4 or 5 weeks old, and now I've added two tiny Sebrights that are about 5 weeks old. They would be totally seperate and not drafty. It's getting down into the 60's and 70's at night here.
If they have their mother to look after them there is no real reason to coop baby chicks except to protect them and their mother from danger. If your chicks are feathered then with or without their mother IMHO they can sleep outside.