Maintaining uniform bird density throughout the broiler house is important year round. Most producers are conscious about putting up migration fences during hot weather when bird migration generally can lead to significant variations in bird density between the fan and inlet ends of a tunnel-ventilated house. However, the use of migration fences is not the norm during the winter months. Even though bird migration may not be as dramatic in cold weather, if birds are not distributed evenly, it can... be very costly, possibly more costly than during hot weather. This is because during cold weather uneven bird distribution affects bird performance and heating costs as well.