Litters of 5 or 6 young are born 19 to 21 days after mating, although females that conceive while nursing may have a slightly longer gestation period. Newborn house mice are hairless and their eyes are closed. Young grow rapidly, and after 2 weeks they are covered with hair and their eyes and ears are open. They begin to make short excursions from the nest and eat solid food at 3 weeks. They are weaned soon after and are sexually mature within 6 to 10 weeks of birth. Mice may breed year-round, but when living outdoors they mostly breed in spring and fall. A female may have 5 to 10 litters annually, causing populations to grow rapidly under optimal conditions. Survival and breeding of young mice declines markedly when densities are high.
Mice may burrow into the ground when other shelter is not readily available. Nests may occur in any sheltered location. Nests are constructed of fibrous materials and usually are 4 to 6 inches in diameter. Mice prefer to nest adjacent to sources of heat, such as compressors, pilot lights, and hot water heaters.
Mice are active year-round, and are most likely to invade structures as temperatures cool in the fall. They are mainly nocturnal, although considerable activity may occur during the day. Seeing mice during the day does not necessarily mean that a large population is present, though this usually is true for rats.
One mouse travels an area averaging 10 to 30 feet in diameter during daily activities. Mice seldom travel far for food or water. Limited movements and food-gathering behavior, which differs from rats, make mice difficult to control in many situations.
Mice leave and return to their nests up to 40 times in a day. They explore, learn, and memorize the locations of pathways, obstacles, food, water, shelter, and other elements in their habitat. They quickly detect new objects but do not fear them. They almost immediately enter bait stations and sample bait. The degree to which mice consume a particular food depends on the flavor and its physiological effect. Mice may reject bait simply because it does not taste as good as other available foods.
House mice live in and around homes, farms, commercial establishments, and open fields. They may be found far from humans, particularly in moderate climates. As a general rule, wherever humans are, mice may be present.
Mice eat many types of food but prefer seeds and grain. They sample new foods and are considered “nibblers.” Foods high in fat, protein, or sugar (e.g., bacon, chocolate, butter, nuts) may be preferred, even when grain and seeds are present. An individual mouse consumes about 1/10 of its body weight each day and up to 8 pounds per year. Mice contaminate far more food than they consume through their urine and feces.
Unlike Norway and roof rats, mice can obtain water from the food they eat. Mice only need about 0.4 ounces of water per day and they often lap condensation on pipes or drips from faucets. They drink water when it is available. Absence of drinking water or food with adequate moisture content may reduce breeding potential.