Expert consulted: 
As temperatures drop, you're not the only one looking for a place to warm up. "Colder weather forces more rodents inside to seek out warmth and shelter," says Dan DiClerico, director of the Good Housekeeping Institute's Home Improvement & Outdoor Lab. "We call it the fall invasion."
Once they're inside, a small mouse problem can grow fast. Female house mice can start breeding as young as six weeks, says Dr. Kathy Heinsohn, B.C.E., a technical and training entomologist at American Pest. Each litter usually has four to eight pups, and a female can get pregnant again within 24 hours of giving birth.
A single mouse can turn into a full-blown infestation in just a few months, which is why it pays to handle the problem fast. Here are the early signs of a mouse infestation, and what to do if you suspect one.
1. You actually see a mouse. (Duh.)
It's the most obvious sign, and the one that should worry you most. "The bad news I give people is that for every one mouse you see in the home, there are usually another 10 or so hiding out nearby," says DiClerico.
Mice are nocturnal, so a daytime sighting is especially worth taking seriously, adds Heinsohn. Daytime sightings don’t necessarily mean you have an infestation, but it could mean a hungry mouse has found an easy entrance point.
2. You find droppings.
Beyond an actual sighting, "mouse droppings are the best indication of an invasion," says DiClerico. "The more droppings you find in an area, the larger the infestation."
Mouse droppings are only about ¼ inch long, roughly the size of a grain of rice. They're usually black, with at least one pointed or pinched end. By comparison, rat droppings can be close to an inch long, says DiClerico.
It can be tricky to identify mouse droppings at first glance because insects, such as cockroaches and crickets, leave evidence of roughly the same size, says Heinsohn. But one way to rule out other invaders is to look at the ends of the droppings. Cockroach and cricket droppings "will both have square ends with no pointed endings," she says.
Droppings that look moist and dark are fresh, and "a definite sign that there are mice nesting nearby," says DiClerico. Just consider where you find the evidence: In dry, dusty spots like attics, even new droppings can look old and dust-colored, says Heinsohn.
3. You notice gnaw marks.
A mouse's front teeth never stop growing, so mice gnaw constantly to file them down. Chewed wires are a big sign, and this is more than just a nuisance. They can short out appliances and even lead to house fires, says Heinsohn.
4. You see grease marks or tracks.
Mice have oily fur, and they tend to use the same routes over and over. Where they repeatedly come and go, they leave "a visible smudge mark," says Heinsohn. In dusty areas, you may also spot tiny paw prints or the trail of a dragging tail, which can show you which direction they're traveling.
5. You come across a nest or a food stash.
Mice nest in warm, dark, and quiet spots. They line their nests with soft materials like string and cotton balls. They also hoard food, so a hidden pile of seeds, nuts, candy or snack crumbs in a drawer or corner is a telltale clue.
Other (less-obvious) clues
A large infestation often has a distinct urine smell, since mice leave frequent drops of urine as they move through their territory. A sudden wave of flies, fleas or mites, or an unexplained bad odor, can also signal that a mouse has died inside a wall, adds Heinsohn.
Where to look for signs of mice
Start in the kitchen. "Mice are drawn to food sources, and they'll eat just about anything that humans eat," says DiClerico. "That makes the kitchen their favorite hangout, especially under and behind larger appliances, or around uncovered trash cans."
From there, check the warm, dark places mice like to nest. This can include corners, radiator openings, wall voids, attics, and the areas around water heaters, dryers, stoves and dishwashers, says Heinsohn.
Focus on the edges of a room. Mice have poor eyesight, so they navigate by keeping their bodies against walls and baseboards and feeling their way with their whiskers. They also tend to stay close to home. "Typically, mice will forage 10 to 30 feet from their primary harborage site," says Heinsohn. So if you find droppings in one spot, the nest probably isn't far.
You saw a mouse—now what?
You don't need to panic, but you also shouldn't wait to take action. These first steps will make your home a lot less inviting for these little critters.
- Cut off the food supply. "If your kitchen is super clean, mice will move on to the next house," says DiClerico. Wipe the counters and sweep the floor every day so crumbs don't draw mice in. You can also store pantry staples in airtight hard-plastic or glass containers with very tight-fitting lids, says Heinsohn.
- Seal entry points. Mice can squeeze through a gap as small as ¼ inch, "which is about the width of a pencil," says DiClerico. Check door sweeps and weep holes, along with gaps around utility and plumbing lines, suggests Heinsohn. Attic and louver vents, uncapped chimneys and crawlspace doors are also common entry points. To find hidden gaps, she suggests a simple light test. During the day, look from inside the house for spots where sunlight shines through. At night, stand outside and look for light escaping from inside. Seal any openings you find.
- Tidy up the perimeter. Mice are good climbers and will use vines, shrubs and tree branches to get inside. Keep plantings near the house to a minimum, recommends Heinsohn. Spreading pea-sized gravel 3 inches deep and 3 feet out around the foundation also makes that area harder for mice to cross.
- Decide whether to DIY or call a pro. "Honestly, it depends on your tolerance for living with mice," says DiClerico. With a clean kitchen and sealed cracks, mice should eventually move on, and traps can handle any that stick around. If you use catch-and-release traps, he says to let the mice go at least a mile from your property, "otherwise they'll find their way back." Some people would rather call an exterminator at the first sighting. "There's nothing wrong with that," he says.
Noah Cortez is a Brooklyn-based writer and editor writing about
travel, culture, food, and lifestyle. Prior to this, he was a social media
editor at GQ Magazine, Complex, and Conde Nast Traveler, and his writing
has been featured in Tripadvisor, Hotels Above Par, Conde Nast Traveler,
and more. Find out more at noahcortez.com.
Having written thousands of product reviews and how-to articles on all aspects of home ownership, from routine maintenance to major renovations, Dan (he/him) brings more than 20 years of industry experience to his role as the director of the Home Improvement & Outdoor Lab at the Good Housekeeping Institute. A one-time roofer and a serial remodeler, Dan can often be found keeping house at his restored Brooklyn brownstone, where he lives with his wife and kids.
