Quite simply, when you turn on an exhaust fan in a poultry house it pulls air out of the house and air from outside the house moves into the house to replace the air the exhaust fan removed. In a house where the side wall curtains are fully opened, the exhausting of this air basically goes unnoticed due to the large amount of air that even the slightest of breezes is constantly moving in and out of the house. For instance, a 36" fan pulls 10,000 cubic feet of air out of the house every minute it... operates. But, with the curtains fully opened in a 500' house, just a 1/4 mph wind will move nearly 50,000 cubic feet of air in and out of the house each minute. Now keep in mind a 1/4 mph wind is basically imperceptible (with a 1/4 mph breeze, 20 ft/min, it would take a full minute to for the air to move half way across a house). If the wind speed increases to just to where you can just start to feel it (two mph), over 400,000 cubic feet of air moves through a house each minute, 40 times what the fan is removing! As you can see, the 36" fan is removing just a small portion of the air that is constantly flowing through the house. In either case the fan does not have to work to pull air into the house because the wind is doing the work for it.