That is a tough one. I don't know how weather ready your birds are.
Some people use a ceramic bulb like is used in reptile cages as they don't emit light.
I know my grown birds will avoid any added heat source. My hubs felt bad for a naked hen one year so put a sealed oil space heater in the giant coop. EVERY bird stayed completely away from the warmer zone.
I would suggest watching for them behaving cold. They will fluff up and sit covering their feet and possibly even shiver when cold.
I have 12 week chicks in an outdoor coop right now. No added heat for about 3 weeks now with lows arpund 15-20° F. The 15°F is about -9 Celsius.