No I don't think the issue was too much heat. I have had chickens for 7 years, and it gets quite warm here during the summer. It is not unusual to have high 90s with high humidity. I know it was not anywhere near 90 in my brooder in the past few weeks when it was near 0 outside. The heat lamp I think was just keeping it above freezing only. My chicks were fully feathered and capable of handling the low temps. BUT every time I attempted to turn the lamp off, they were in the dark and I think they panicked. I think they were climbing on top of one another and smothering the weaker chicks. In the beginning I had a couple of chicks die but think that was something wrong with them. The last 3 that died I think was due solely to panic.
The biggest reason why I have decided to go with the cave method is because I think it is much closer to "momma". There is a definite day/night schedule. There is no constant artificial light. Chicks can come and go from the warmth as they decide that they need it. And as they get older I can slowly turn down the temp so eventually they have no extra heat. There will be no panic when suddenly the light is just turned off.
Looking out the window at my girls right now, they are all in the run doing exactly what chickens are supposed to be doing. NOT locked up in a box with light blaring down on them.
I have learned a lot caring for chickens over the last several year. I started out with day old chicks. I brooded them with a light and then moved them outside in early summer. I lost bird after bird to predators until I finally figured out how to protect them appropriately. I have not had a predator death in over 3 years. Then I stared hatching. My hatch rates were terrible... 30%... I was so frustrated that I almost gave up. That's when I found BYC and all of you wonderful people helped me figure out what I was doing wrong. Last year my hatch rate was consistently between 87%-94%. I have finally figured out what works there. Now it's brooding. I lost 5 chicks and that really sucks! But the best part is that now I know what I was doing wrong and have learned what to do right!!!