Norway Rats: Identification, Signs & Risks

The Norway rat (Rattus norvegicus), also called the brown rat or sewer rat, is the rat you're most likely to see in the Chicago area. Like the house mouse, it isn't native to Illinois. It came from Europe and lives wherever people do.

• Size: a heavy body 7–10 inches long, plus a tail that's slightly shorter than the body. Adults usually weigh 7–18 oz.
• Color: coarse brown or grayish-brown fur on top, with a lighter gray belly.
• Features: a blunt nose, small ears close to the head, small eyes and a thick, scaly tail.
• Habits: Norway rats dig burrows in the ground, swim well and are most active at night.

Rats eat almost anything: grains, pet food, birdseed, garbage, dog waste and fallen fruit. A female can have four to six litters a year, with around seven or eight young each, and young rats can breed at about three months old. Chicago has ranked at or near the top of national "rattiest city" rankings for more than a decade, and the suburbs see plenty of rat activity too.

The roof rat, which climbs and nests high up, is common in southern and coastal states but rare in northern Illinois. If you have rats around Chicago, they're almost always Norway rats.

Rats are looking for food, water and shelter, and they find all three around homes and businesses. A rat can squeeze through a gap about ½ inch wide, roughly the size of a quarter, and it can gnaw through wood, plastic, soft metal and even poor-quality concrete to make a gap bigger.

Outdoors, Norway rats usually live in burrows along foundations, under decks, sheds, patios and garden beds, and near trash cans or compost. From there they move into garages, basements and crawl spaces, especially in the fall and when winter arrives.

• Foundation cracks and gaps where pipes, cables and A/C lines enter the building
• Garage doors with worn bottom seals or gaps at the corners
• Vents without screens, including dryer and crawl space vents
• Sewer and drain lines, including broken pipes and floor drains without covers
• Gaps under doors and around basement windows and window wells

• Droppings: dark, capsule-shaped and blunt at the ends, about ½–¾ inch long
• Burrows: holes 2–4 inches wide in soil along foundations, under decks and beside sheds, often with smooth, packed runways leading to them
• Gnaw marks on wood, plastic, trash cans and wiring
• Rub marks: dark, greasy smudges along walls and pipes where rats run regularly
• Noises in walls, ceilings or crawl spaces at night
• Pets acting excited or pawing at a wall or under an appliance

Not sure if it's a rat or a mouse? See our house mouse page for a side-by-side comparison.