Overheating can occur at the hatchery, during transport in hot weather conditions, or in the brooder house. It occurs when temperatures are too high (both environmental and brooding temperature), when there is a lack of ventilation or when chick boxes are stacked too tight.
Birds are panting, have outstretched wings, are sweaty and trying to get away from the heat. Mortality will follow due to starve out / dehydration. The underlying cause should be resolved as soon as possible and electrolytes provided to aid recovery.