.....and.....
Where in this world are you located?
Climate, and time of year, is almost always a factor.
Please add your general geographical location to your profile.
It's easy to do, and then it's always there!
No concern with predators?
Do they have somewhere to shelter out of the wind/snow/rain?
How big is your coop, in feet by feet?
Dimensions and pics, inside and out, would help immensely here.
They do need ventilation, even in winter, but I seriously doubt they need heat.
Where in this world are...