Anything less than 65 degree's without having protection from the outside air, like a deep, warm nest box or the like in a building might kill them if they didn't have a slow, over time ability to get use to the change slowly. In other words they must be acclimated slowly over time. This and the following temp's are for Healthy Birds Only.
Sick or in poor health birds would die if not kept warmer than normal.
Heat is a must for sick birds.
Larger avian species can survive when acclimated over time to 60 degree's IF they have protection from cold and winds.
So, for my birds, the answer to the question is going to be 65 to 85 degrees. I think...