I have not seen my little ones under their mama during the day since they were one week old, and we have had a cold December with highs usually in the 40's and some 50's lows in the teens several nights. No heat, no insulation, and an airy coop. It is 28 as I type at 8:35 AM; I just returned from giving them some unfrozen water.
I brooded two sets of chicks in the house a few years ago (never again) and kept temps about 10 degrees or more below those listed by gryeyes (and recommended here,) because when it was warmer than that, they would stay at the other end of the brooder -- and this was a large L shaped brooder. I had to raise the heat that high to get them to use the whole brooder.
Old timers around here would hang a 100W bulb in a box outdoors and call it a brooder. Occasionally they lost some to piling on a really cold night, but most survived. Now, admittedly, these old timers also call cracked corn chicken feed; just talked to one the other day, and he works in a feed store.... Not saying the old ways are better -- just that some of our new ways may not be, either.