Mine rarely spent any time inside the Little Monitor Coop when they could be outside despite the excellent ventilation. They even went out on rainy days at 40F.
But not eating enough during short days is certainly an issue that should be addressed.
Don't worry, 0C isn't cold to a chicken because they are wearing built-in down parkas. Chickens are almost always perfectly fine down to 0F -- about -17C.
As long as they have the option of going into a place that is dry and out of the wind a fully-feathered, adult chicken can be trusted to...