Acclimation is very important. I do not provide supplemental heat to adult birds as it interferes with their ability to become acclimated to the ambient temperatures.
This year the ambient temperatures they acclimated to and did fine with were from -40°F to 103°F. Fortunately it didn't happen overnight but a 40°F to 60°F temperature change on a daily basis is not uncommon.
It was -40°F on the morning of Thursday, Dec. 22. It was 47°F the afternoon of Saturday, Dec. 24 for an increase in temperature of 87°F over a period of about 54 hours.
When the cold front blew in there was a drop from 32°F to zero in about 1 hour.
All of my poultry lived through it fine.