There is a website that lists the breeds and all about cold/heat tolerance. It is the Henderson's Chicken Breed Chart
http://www.ithaca.edu/staff/jhenderson/chooks/chooks.html Maybe that will help. I have my coop and run in the shade. I have lots of open sides (with wire) with roosts where they can catch the breeze. They also free range and go into the bushes in the summer. So far, the heat hasn't harmed any of the birds!
Thanks so much! I had used that site to do some of my research but had lost the link...now I have it again
