Many of us have successfully brooded chicks outdoors this past year using the heating pad/cave system. Unless your low temps are down in the minuses, you might look into the heating pad as your heat source. That way, your chicks will regulate their own heat needs, and the rest of the time, they can play in the dirt is their tractor run.
See the thread here on this forum, "Mama Heating pad in the Brooder."
See the thread here on this forum, "Mama Heating pad in the Brooder."