How do you know if you have rats or mice, or some other pest problem? Correct pest identification is important for deciding on an appropriate course of action.
Visual identification is not easy because rats and mice are nocturnal, meaning they are most active at night.
Checking for them after dark with the aid of a flashlight can be successful. If rats or mice are seen during the day, that’s a real cause for concern, because it’s an indication of a serious rodent infestation (or food is very scarce, or they’ve been poisoned and are sick).
Here are 7 identifying signs for rat and mouse activity to look for:
1. Rodent Droppings
- Mouse droppings are small, about 3mm (1/8”) and less than 6mm (1/4″) and pointed on both ends.
- Rat droppings are around 12mm (1/2”) in diameter and length, or larger. The droppings of Norway rats are blunt at both ends, while the droppings of Black rats have pointed ends.
- Older droppings are dull and grey, dry and tend to crumble when cleaned, while fresh droppings are softer, shinier, darker, and are an indication of recent rodent activity.
2. Rodent Nests
- Mice use fine, shredded fibrous materials such as paper, fabrics and string (which they gnaw) to build their nests in indoor locations such as garages, closets, attics, basements, and outbuildings.
- Black rats usually build their nests above the ground, in attics, trees, dense vegetation, between large garden pots or around cluttered areas. Nests are constructed of leaves and other debris, with gathered food stored there too.
- Norway rats build their nests outdoors, constructing burrows around 45cm (18″) deep in the ground, under piles of rubbish, concrete slabs, along walls and in earth banks. Nests usually have a runway leading to a 7cm (3″) opening, with freshly dug dirt scattered in front of it.
3. Rodent Trails
Rats and mice are creatures of habit, they take the same routes when leaving their nests to travel to sources of food and water. These pathways that they frequently travel along are referred to as rat and mouse runways.
Runways usually follow straight paths along natural or manmade edges and lines. These paths running along structures or objects are safer than wandering through wide-open spaces, where they would be more vulnerable to predators.
- Indoor runways may follow along the baseboards of interior walls, over attic roof beams, indoor pipe runs and around or behind large objects such as storage boxes and equipment.
- Outdoor runways may follow building foundations, deck and sill ledges, electrical wires and conduits, fence rails, pipes, sewer lines, tree branches and wall tops.
Mice are quite timid and will usually only travel distances of 2-9m (6-30′) from their nests to a source of food.
Rats are far more adventurous and will travel much longer distances of 8-30m (25-100′) from their nests to a source of food.
Outdoor paths that are being used as rat runways will be identifiable as trails around 5-7cm (2-3″) wide, that are smooth, with the ground packed hard and flattened beneath the vegetation.
Indoor paths usually follow walls, and the presence of undisturbed dust and cobwebs along them indicate that rats or mice are NOT present there.
The tracks left by rats and mice, namely, their footprints or tail marks, may be seen on dusty surfaces indoors, or muddy areas outdoors.
To detect if rats or mice are circulating indoors, make a tracking patch (or several) by lightly dusting a fairly thin, even layer of talcum powder or flour in a patch along a suspected pathway and leave it overnight to determine if rodents are present.
The footprints of rats have a 4-toe front foot and a 5-toe rear foot. They may also leave tail drag marks between the footprints in the powder.
4. Rub Marks Along Rodent Runways
Rats and mice use the same runways between their food and water sources and their burrows, and they follow odour trails to navigate them. When they travel along their runways, their body makes contact with the ground, leaving a scent or odour trail on their runways which they can follow.
Rats have greasy fur, which usually leaves dark greasy marks and a slightly greasy path along their runway. Rub marks on walls appear as black smudges. When rub marks are fresh, they are soft and smudge, and can be smeared if rubbed with a cloth. The grease of old rub marks, found on older runways that are no longer used, becomes brittle and will flake away when scratched.
Mice also leave rub marks from the oil rubbing off their fur onto surfaces along their preferred routes. These marks created by the oil will stain wood, drywall, and many other materials. Mice will leave marks along walls, beams, rafters, pipes, baseboards, and other household surfaces. Being smaller, their rub marks may be less noticeable than those left by rats.
5. Trails of Urine Along Rodent Pathways
Rat and mouse urine smell identical, both have the same strong musky odour combined with an unpleasant hint of ammonia, which is easy to smell when there are large rodent infestations.
Other persistent musky odours may indicate the presence of house mice, as they have a characteristic musky ‘mousy’ odour (which is different to a urine smell), that rats don’t have. Owners of pet mice are aware that mice have a particular smell, which can be quite pungent, and that the males are smellier than the females, as they use scent to mark territory, like many other animals.
Researchers have found that male house mice produce many small urinary scent marks for communication, and chemical proteins in their scent marks convey genetic information such as species, sex, and individual identity, as well as metabolic information such as social dominance, reproductive and health status
Rodents urinate while running, leaving characteristic streaking stains along runways. They may also urinate in areas where they feed. Both wet and dry urine will fluoresce under ultraviolet (UV) light (as will some other materials), and fresh urine will glow a bright blue-white under UV.
Specialised UV (blacklight) LED flashlights for locating pet stains, available from pet shops, can be used for this purpose. Avoid the cheap ones that use 395nm LED lights, they don’t work well, as they produce lots of visible violet-colored light. The better UV flashlights use a 365nm LED light, which is more in the UV-A range, and emit a dull, bluish-white light, which will illuminate stains and other materials much better, as they don’t emit violet light, and many objects fluoresce the most at this 365nm wavelength.
That’s probably more about rodent urine than most people care to know!
6. Gnawed Objects
Rodent’s paired incisor teeth grow continuously, so they need to gnaw on hard surfaces every day, and work them against each other to keep their teeth short and sharp. They also gnaw holes to gain access to new areas and to reach food.
Gnawing marks may be observed on various surfaces where rats and mice are present. These include containers of stored materials, food containers and foods in the pantry, wall materials, electrical wires, as well as the edges of doors, ledges, posts and beams.
In the garden, rodents will gnaw on fruit, vegetables and nuts, and will chew holes through garden netting which normally keeps other pests out. If something is making holes in your garden netting, it’s likely a rat or mouse!
Fresh piles of wood shavings, roof insulation, or other gnawed materials indicate rats or mice are active.
Rats and mice will also gnaw holes in walls. We can distinguish which it is by the size of the entry holes and the size of the teeth marks.
- Mouse entry holes are usually around 4cm (1-1/2″) in diameter or less
- Rat entry holes are 5cm (2″) or larger
7. Sounds from Wall and Rood Cavities
Rats and mice make sound when they’re moving around, scratching or gnawing, that are usually heard at night or in quiet areas. Their activity can be heard in ceilings and walls. They also make sounds when they fight, and their young often make squeaking sounds while in the nest.
Larger animals that get into the roof will make much louder noises when moving around because of their greater size.
Once you know you have rats or mice, you can then take the appropriate course of action to control them.
Which Rat and Mouse Control Methods Really Work Best?
Two of the most common rodent control methods are baiting and trapping.
The danger with using commercial poison baits to control rats and mice is that pets or native wildlife which eat these poisoned rodents may be harmed.
A better baiting option for rats and mice is an environmentally friendly, home-made bait that uses bicarbonate of soda (baking soda) which exploits a unique feature of rodent biology, the inability to burp or vomit. For more details, see the article:
Trapping rats and mice using snap traps is highly effective and very cheap. For instructions on the best ways to trap rats and mice, see the article:
More articles on Garden Pests, Diseases and Problems
- IPM Handout for Family Child Care Homes, INTEGRATED PEST MANAGEMENT: RATS AND MICE, California Childcare Health Program, University of California, San Francisco School of Nursing
- Rat and Mouse Control – ENY-224, UF/IFAS Extension Service, University of Florida, P. G. Koehler and W. H. Kern Jr.
- University of California, Statewide Integrated Pest Management Program, How to Manage Pests – Pests of Homes, Structures, People, and Pets – House Mouse, Revised 10/11
- Prevention and Control of Wildlife Damage. Editors, Scott E. Hygnstrom, Robert M. Timm, Gary E. Larson. 1994. University of Nebraska-Lincoln. 2 vols., House Mice
- Arakawa H, Blanchard DC, Arakawa K, Dunlap C, Blanchard RJ. Scent marking behavior as an odorant communication in mice. Neurosci Biobehav Rev. 2008 Sep;32(7):1236-48. doi: 10.1016/j.neubiorev.2008.05.012. Epub 2008 May 15. PMID: 18565582; PMCID: PMC2577770.