I have 3 five week old keets and they are in a brooder in my basement. They are getting kind of big for it and I'd like to put them in their coop but it has been getting down in the 30's at night. The coop is actually a run in shed with an open front but the front wall is just hardware cloth so I can't really keep them warm enough with a heat lamp. When do you think they should be feathered out enough to handle the cold?