So how do wild birds grow down feathers sufficient to survive in your constantly changing temps? Because they do. If your local birds are capable of it then birds that originated in northern china, southern russia, and the mountains of japan can surely handle it.
Eta: I was also curious how the Canadian members are able to leave their birds in unheated coops with nothing more than plastic sheeting to block draft, and them still survive. It gets much colder in Canada than Chicago.
Sure healthy cold tolerant birds can handle the cold temps, never suggested they could not... But, fact is even with native animals that can technically handle the weather in their region, many still parish every winter due to exposure... In regards to birds that people keep not everyone limits themselves to birds suitable for their regions temps...
Think of it this way....
If someone proclaimed a blanket statement that dogs don't need a heated dog house and will be just fine in the winter without one just like feral or wild K9s are, is that a true or false statement?
I believe most would agree that it's neither true nor false and instead the answer fully depends on the breed of dog in question... The same applies to birds they are not all created equal...