Nighttime cooling is an important aspect of maximizing bird performance during hot weather. Even in the best houses there will be days when the birds will become heat stressed. If the ventilation system is well designed and maintained, increased bird mortality will not typically become an issue but weight gains and feed conversions can be adversely affected. How much performance will be affected on these days will depend to a large extent on how the ventilation system is managed at night. If the... ventilation system is managed properly, bird performance will be minimally affected by hot weather. But, if the ventilation system is not managed properly birds can actually be made to feel hotter at night than they were during the day, thus aggravating hot weather performance problems.