The house mouse is often referred to as the "Mammalian Weed." It is found everywhere people are established, as well as in some areas people are not. Although house mice may be a problem year round, the peak activity of invasions into the homes occur during the autumn months. People often assume that mice come into our homes to get in from the cold, but this is only partially true. With the onset of the colder months, the natural foods (insects and plant seeds) and harborage (vegetative cover) of mice begin to decrease. The mice then begin to forage about, seeking environments where food and warmth will still be readily available. While foraging a lucky mouse may randomly encounter a gap beneath a garage door or some other area in which heat or food odors are emanating.Gaining entry into homes is not difficult for a mouse unless it is a new home, or one that has been pest-proofed. It is not true that mice can completely flatten their bodies, or that they have special skeletons with flexible joints or bones. For a mouse to get through a hole, the opening must accommodate the height of the skull of the mouse, which measures one fourth of an inch. The rest of the body easily follows. Often, once an opening is used, it will be found and used by other mice in the area unless the opening is repaired. This is because the first mouse through the opening may deposit body secretions and urine, resulting in an odor trail (mouse pheromone trail).
Indoors with continuous food and warmth, the house mouse may breed all year long. The average litter is five to six pups following a 21 day gestation period. Within about three weeks, the pups are foraging outside their nest on their own, and they can reach sexual maturity as early as six weeks of age. Thus, an undetected family of six mice, in a cluttered garage with dog food, can expand to 50 to 60 mice within only 90 days.