Mine are outside all year.I do poly the sides for the winter and the top is always covered,but no heat.They do great and are healthy little birds.You just need to keep the drafts off from your birds and they will thrive.
In N.H.,Tony.
For their size, they can handle very hot and very cold temps pretty well, as long as they are kept out of direct drafts. If they can get out of the wind and the weather, and have plenty of food and water, they'll stay warm
I too have kept DDs outside before, and our winters get much colder than that. This morning when I got up, it was 17 degrees