So the bad news is you've got field mice. The good news is they are outside the house. When the weather starts to cool, though, they will do everything in their power to find a way indoors, so your best strategy is to get rid of them first. There are a variety of ways, lethal and non-lethal, to get the job done.
Get an outdoor cat -- a good mouser -- and let it hunt the mice. An indoor hunter will take care of any mice that get inside.
Set out containers of rat poison in areas where pets and children won't be able to come in contact with them. If there is any possibility children and family pets can access the containers, don't use poison.
Use a live trap, which can catch 20 or more mice without killing them. The mice must then be removed far from the home and set free so the trap can be reused.
Bait mouse traps with peanut butter and leave them in barns and other outbuildings. Don't leave traps in open areas -- the peanut butter will merely attract more mice.
Eliminate nesting areas for mice by removing brush piles and by moving woodpiles away from buildings. This will provide field mice with less protection from predators and the elements.
Eliminate potential food supplies such as bags of grass seed or pet food, especially accessible feed or seed stored in garages, barns and outbuildings.
Build a nest to attract owls. A plastic container with a 17.5 cm (7 inch) hole cut in the side and filled with twigs and straw can be tied to the upper branches of a tree on your farm or open land.
Place chunks of steel wool in every hole, crack and crevice you can find around the outside of your home. This provides a low-cost barrier to field mice trying to find their way inside.