We're in AZ so our brooder is outside! We currently have chicks not even a week old yet. We just have 18 right now. We have a big dog house in our coop with a 100 watt bulb in it for warmth. BUT we have in our brooder, ducks, chickens, and turkeys! I read somewhere that as long as you have a certain number chicks & chickens to a certain amount of space they all keep each other warm when they huddle to sleep. This has worked for us so far, and were currently on our 4th wave of chickens! ( 3rd wave of ducks, and first wave of turkeys) Yesterday it was 75 here, and today 83*, at night it's been in the 40's. We don't live in the city, we're in the middle of NOWHERE!
Hope this helps!