It seems they can endure all sorts of discomfort; endure being the operative word.
The cold hardy reasoning from what I've read is mainly based on they didn't drop dead; well not very many anyway.
People happily assume that a chickens feathers are there to keep them warm. What is a bird whose ancestors lived and live in a temperature range of 25 Centigrade (77F) to 38 Centigrade (100F) doing wandering around with a thermal quilt on? They don't need to be kept warm in those temperatures.
Obviously something is wrong with the reasoning process.
Over several millennia of selective breeding humans have transformed jungle birds into hardy, cold-tolerant birds to the point that many of the breeds developed for cold-winter areas suffer and fail to thrive in the heat.
In my flock, when the temperatures climb over 95F (35C), and the humidity climbs with them the Wyandottes -- a breed developed to withstand the bitter cold winters of the US Northeast -- wilt and droop, becoming far less active and retreating to the deepest shade available. I haven't lost one to the heat (having taken the precaution of sourcing them from a hatchery in Texas, where the climate is even hotter than mine), but if I ever lose birds to the heat I will expect it to be the Wyandottes.
I would not keep an even more cold-selected breed like a Chantecler in my hot climate and I wouldn't keep Jungle Fowl in New England where I used to live.
If the chickens are healthy and active, going out and about and engaging in their natural behaviors, they're fine.
