Agreed, they need to get used to the cooler temps, no matter how long they've been fully feathered. You can put them outside once you can provide the proper, dry, draft free temperatures, and you might accomplish this with varying sized/wattages of heat lamps or brooder heaters. Last year I used two, 250-watt lamps and a 400-watt brooder heater to step them down from the 60-degree range or so (don't recall exactly), to the 10 or 20 degree lows we were having at the time. It took forever but I didn't lose any of the 27 chicks. Also indispensible, was the chance to do a "dry run" of the brooder before the chicks came. First, I found that the chosen coop actually was totally inadequate, so I created a brooder corner in the shed that worked. Then built a new coop and tested that. This year I found that my brooder heater decided to retire, the day before the chicks came.