I put a lamp in over a section of their roost area. If they are cold they will move over to be close to the heat source. Mine seem to all huddle together no matter what the temp is.
my chicks do this also, hot, cold, rain.... they all huddle up in the corner of the coop..... I think its just a natural chick thing. Kinda funny to watch them fight about who's gonna lay closer to the door !!! why they cant use a box is beyond me (the use to) even funnier when I open the door and chicks come falling out, silly chicks !!!