Outside storage question

I have a storage problem and will need to move a tractor or two to my pole barn.  These are restored tractors.  Any ideas on how to best keep them in a pole barn and getting infested with critters or rusting from exposure?  

Any type of tarp or cover, even if tied securely, will rub on the paint and scuff it in places, if the wind has the ability get at the tractor.

When we moved over the last year I had to store some of my restored treasures in less than perfect conditions and I learned a bit. As Doc said dont cover them, dirt and poop washes off, tarp damage does not and tarps hold moisture in. Just let them get dusty and dirty and wash it off when you can put them back in good storage. Dont wash the tractor spray some type of oil on any shiny metal that you don't want to rust.  Leave the tool box open and mice will not nest there. Do cover up the clutch and brake housing inspection holes. 

I had 100 yards of gravel hauled in and leveled it off. A little slope in each direction so I now have a well drained gravel parking spot for every thing that sits outside. Any time you park over grass your parking on top of a moisture pump. As the grass or weeds in my case grow they wick water up. Plants have to have water to create energy. When they touch metal invariably some sap is transferred to the metal just by the wiping action when the wind blows. Sap is a slightly sugary water solution and it causes rust. If you have to park over grass move it every couple weeks and mow it short.

In any places mice would want to nest throw some moth balls

Thank you all for these suggestions.  I never thought about putting a vapor barrier under the tractor, but it makes sense. 

you can hang a tarp up on the bottom side of the rafters, letting the sides hang down about 2 ft all around. that will keep some of the bird poop off as they will sit on the rafters and the tarp will catch that part, and the waving and fluttering of the tarp edges will keep most of them from landing on the tractor to poop!