Use duct seal to keep mice out. Rodents love to nest inside electrical equipment and chew up the wiring. Generators are particularly vulnerable because of heat from the coolant heater. DUCT SEAL is a plastic putty much like Play-Doh, only not edible. You can get it at any electrical supply house.
Plug all open conduits that enter from outside. Mice will follow the conduit into your building, transfer switch, or generator and make a nasty mess! Check out this YouTube video: Monthly Generator Operational Inspection & Testing.