We're in the 40s-60s F here right now, no tarps or heating yet. Once it approaches freezing, I tarp up the aviary's smallest section and let the birds decide if they want to be in or out at any given time.
Add about a bale's worth of straw (per 30-ish square feet) over the winter, keeps them insulated and their feet stay dry and clean. Give them lots of cramped hiding places that will trap their heat—they seem to know they're better off resting there than in the open, except for night time when they learned to huddle up.
Last year, supplemental heat in the form of heat pads was necessary for my oldest birds' safety and the whole flock's comfort. Got down to about five degrees Fahrenheit some nights, but birds were fine come spring.