My latest batch of Silkies is about a month old, and they do just fine at night (here in VA, its been anywhere from 65-70°F at night) without a heat lamp. BUT- there are 5 of them and they do cuddle up in their little brooding box to keep warm. Could you put a heat lamp in a corner of their outside pen? If not, I've found a good sized piece of glass (like from an old window or car) leaned against a wall in a sunny spot makes a nice outside warm up spot (think greenhouse). My little chicks love to get behind their old car window and soak up some sun. Good luck!