The presence of house mice can be determined by a number of signs described below:
Droppings may be found along runways, in feeding areas, and near shelter. Differentiating between mouse droppings and those of certain insects may be difficult. Mouse droppings are about 1/4 inch (0.6 cm) long, whereas those of cockroaches are usually 1/8 to 1/4 inch (0.3 to 0.6 cm) long and under a magnifying glass show distinct longitudinal ridges and squared-off ends. In comparison, droppings of bats contain insect fragments and are more easily crushed between the fingers.
Tracks, including footprints or tail marks, may be seen on dusty surfaces or in mud. A tracking patch made of flour, rolled smooth with a cylindrical object, can be placed in pathways overnight to determine if rodents are present.
Urine, both wet and dry, will fluoresce under ultraviolet light, although so will some other materials. Urine stains may occur along travelways or in feeding areas.
Smudge marks (rub marks) may occur on beams, rafters, pipes, walls, and other parts of structures. They are the result of oil and dirt rubbing off mice’s fur along frequently traveled routes. They may be less apparent than rub marks left by rats.
Gnawing may be visible on doors, ledges, in corners, in wall material, on stored materials, or on other surfaces wherever mice are present. Fresh accumulations of wood shavings, insulation, and other gnawed material indicate active infestations. Size of entry holes (often 1 1/2 inches [3.8 cm] in diameter or less for mice, 2 inches [5 cm] or larger for rats) or tooth marks can be used to distinguish rat gnawing from mouse gnawing. Mice keep their paired incisor teeth, which grow continuously, worn down by gnawing on hard surfaces and by working them against each other.
Sounds such as gnawing, climbing in walls, running across the upper surface of ceilings, and squeaks are common where mice are present.
Visual sightings of mice may be possible during daylight hours, and mice also can be seen after dark with the aid of a flashlight or spotlight.
Nests frequently are found when cleaning garages, closets, attics, basements, and outbuildings where mice are present. They consist of fine, shredded fibrous materials
Odors may indicate the presence of house mice. A characteristic musky odor is a positive indication that house mice are present, and this odor can be used to differentiate their presence from that of rats.
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