Most mammals perspire when they are hot, and evaporation of this perspiration from their skin is extremely effective in reducing body temperature. Nevertheless, birds have a couple of special features that do help them during hot weather. Their relatively high body temperature makes it easier for them to lose heat to the air around them. Also the bird's respiratory system is very effective at cooling. Thus panting in birds is analogous to perspiring in mammals and is extremely effective at cooling the bird.
Air movement is critical.
A bird can only give off heat to its environment if the temperature of that environment is cooler than the bird. If heat produced by the birds is not moved away from them and out of the poultry house quickly, it will be more difficult for them to avoid heat stress. The best ventilation systems provide at least one air exchange per minute. Also, it is critically important to continue moving air during the cool parts of the day and night. When birds can get relief from the heat at night, they perform much better.