Because you have been heating them all along, you will need to wean them off of it. I do not provide my guineas with any supplemental heat or light. It can and does get down to -30ºF here.I have a heat light in coop at night but never in the box. I was told they need this heat lamp due to frigid temperatures. They won’t freeze in below 30º temps?
Adult guineas do not need supplemental heat. Providing them with supplemental heat has prevented them from properly developing their feathers to protect them from cold temperatures.