Mice and Rats

The natural habitat of wild mice and rats includes forests and grasslands, and their range extends from sea level to high mountain elevations.

Known as commensal, meaning "sharing the same table," some species of mice and rats have a long history of living close to people. Fossil records place evidence of House Mice in a Neolithic Turkish community more than 8,000 years ago.

The spread of mice and rats coincided with:

The construction of houses and barns, which offered them shelter.
The development of agriculture, which provided them food.
The beginning of ship travel, which hastened their spread over water.

Wild mice and rats are omnivores, eating a variety of vegetable matter, insects and meat. Commensal rodents feed on any human food available, as well as birdseed and pet food.

House Mice

House Mice range in color from light brown to black, usually with lighter underparts. Domestic strains of House Mice are predominantly white with black or gray markings.

House mice build loose nests of shredded paper and fabric behind rafters, in woodpiles, in storage areas or other hidden locations where there is a nearby source of food.

They breed continuously and have five to 10 litters per year. Litters average five to six young who are born blind and without fur. Fully furred after 10 days and weaned at about three weeks, House Mice reach sexual maturity between five and six weeks. Generally, there is a high mortality rate of young mice, and the average life span of adults is one year.

Norway and Roof Rats

The most common species of commensal rats are Norway and Roof Rats. Both are found in cities and suburbs where food and shelter are plentiful.

Norway Rats are ground dwellers and often live in cellars, basements and lower floors of buildings. Because they swim well, they can survive in sewers.

Roof Rats differ from Norway Rats by their darker color, slender bodies, pointed muzzles and tails that are longer than their bodies. As their name implies, they are agile climbers and are often found on roofs and in the upper levels of buildings.
Commensal rats may reproduce one to 12 litters in a year, with average litter size of eight to nine. The young leave the nest at about 20 days, and they reach sexual maturity at two to three months.

Solving and preventing conflicts

Because they are gnawers, mice and rats can cause damage to buildings, especially when they chew through insulation and wiring. Rodents are attracted to stored food, especially grain and seed, and they can contaminate it with their urine and feces.

Property owners can discourage and exclude rodents by creating an unfriendly habitat.

Keep rodents away from food

Follow these steps to protect food:

Store food in secure containers.
Dispose of garbage in metal cans with tight-fitting lids.
Do not put food scraps on open compost piles.
Bury food waste in an underground composter or in a lidded worm box (read more about composting from Seattle Tilth).

If you feed birds, store seed in secure, metal containers. Put only as much seed in feeders as will be eaten in a day, and clean up spilled seed and hulls immediately.

Keep rodents out of your home

Rodents can enter buildings through holes as small as one-half inch in diameter and they can usually climb interior walls. You need to seal your house and other buildings thoroughly.

Use hardware cloth with squares one-quarter inch or smaller to patch larger holes, screen vent openings, and skirt building foundations at least 12 inches underground.

Stuff steel wool into cracks and around drainpipes and other small openings.
Seal the stuffing with expanding spray foam, caulking or cement.
Seal all holes and cracks in walls, eaves, and roofs.
Inspect your home regularly and repair any potential entry points.

Make your yard unattractive

Rodents also nest outdoors, especially where overgrown plants offer cover. You can eliminate their shelter sites by:

Mowing long grass.
Removing weeds and brush.
Trimming shrubs at ground level and large ground cover plants such as ivy.
Removing debris and woodpiles close to buildings.

Rodents are also drawn to pet food. It is best not to feed cats and dogs outside, but if you have no other choice, pick up bowls, spilled food and leftovers promptly.