I have always believed that one of the truly great qualities of our old and rare breeds is the fact that they came from different regions of the world with different climates. Thus, we are able to make breed choices based on our on climates.
Here in the South it gets extremely hot as you know, but the 6" of snow outside my window also speaks to the extremes of our weather. We can range from a low of 15F to a high of 105F+.
I have found that the British breeds really suffer in our weather.
The French, Games, and Oriental breeds tend to do the best I've found. (Or breeds with some Oriental background such as the second oldest American breed the Java).
It is not our cold the birds have the most trouble with (as we are seldom cold). It is our extreme heat and humidity that can last for days and days. In July and August we will have many nights that never fall below 80F and the birds just don't have a chance to recover. Excessively feathered birds just have a very hard time of it; so, I don't keep them.