Hello!
I am not sure that this will answer your question, but I hope it might give you some insight. I have 30x 4 week old Jumbo Cornish Cross that we placed in our chicken tractors at 3 weeks old (without a heat source). So, they've been outside for about a week. Our daytime temps are 80-95 and night temps have been in the upper 60s and 70s.
We did not take any losses our first week outside (or since they arrived).
Yesterday we had high temps (95ish) and I could tell they were HOT.
Last night we had a big thunderstorm move through and the lightning was close and the thunder was LOUD. When I went to move the tractors this morning -- one was deceased. No outward signs of damage -- and I think it likely died of a heart attack from fear/stress.
I am not sure if it was the heat from yesterday that did it in, or if it was the thunderstorm - but I suspect the thunderstorm is to blame.
The reason that I share this information with you -- is because if you think it's safe (temp wise) for your birds to be outside -- then it may be that the stress of moving outside plus the thunderstorm did them in. Perhaps after they have time to adjust to the tractor, they may handle the storms better in the future?
I can't say for certain - but - I have attempted to provide you with as much detail as I can about my situation so that you can determine if this matches your experience. I will attach some photos of our chicken tractor setups so that you can see how we provide the cover to them in the tractors.
I hope this helps in some small way and I'm sorry about the loss of your birds.