Hello, I have 3 six-and-a-half-week-old chicks (light brahma, barred plymouth rock, amauricana) who are starting to be crowded in their brooder, and we're working on building them a coop right now. I followed the recommended temp charts for them as they grew, but now that they are over 6 weeks old and fully feathered, I wonder what the common wisdom is on when they stop needing supplemental heat. It is December 1st, and I'm in Austin TX. It looks like we'll get a cold snap in a few days that will bring nighttime lows in the low 40s. This is too cold for them, even if they sleep huddled, right? When are they old enough to tolerate temps in the 30s-40s without a heat lamp? I can run the heat lamp out to the coop, but since we're just headed into winter, I'm wondering how long before they won't need any supplemental heat. Thanks!