I live in upstate NY. I have 18 Buff Orphingtons, and 10 Cochin Bantam chicks I'm brooding in a 5 x 4 brooder. To say it's crowded is an understatement. I want to move the orphingtons (they're half feathered already) to their coop. It's 17 x 8. They would have more room and be secure there. Lots of Windows for light etc; Will they stay warm enough if it gets into the 40s at night? I can run an electric cord out at night for a heat lamp...how big should it be?