I've found that leaving the hood open on my 2520 has kept them from nesting in there. The previous winter I tried keeping it closed and would pull a nest off of the top of the fuel tank almost every time I used the tractor. "Bounce" dryer sheets seem to work for me. One spring I fired up the JD 175 Hydro for the first time in the shed and heard something bouncing off of my boat. After looking I found what the noise was, dog food was flying out of the exhaust. I took the tractor outside and started mowing, I had "smoking dog food" coming out for about the first 10 minutes I mowed.