Heat has stopped being a seasonal annoyance for poultry farms. As the planet warms, broiler flocks are spending more of their lives just trying to stay cool instead of putting on weight. Most of what we knew about heat stress came from studies that baked birds at a fixed high temperature all day and night. That is easy to run in a lab, but it barely looks like a real farm, where the worst heat rolls in during the afternoon and backs off after dark. I keep coming back to one trial run in Thailand, because it tried to be honest about that gap. Instead of a constant oven, it…