Been in the 20s here lately and my birds are fine. Last summer they survived low triple digits.
Heated water, proper ventilation, and all access to feed I blame it on.
Windows are nice. I ended up cutting one in mine just for added light. Mine is nothing fancy, just some plexiglass I had and used 2x4s for the frame. They don't open. Just an source of extra light so I'm not wasting electricity.