What Are the Signs of a Rat Infestation?

Droppings are the anchor sign: Norway rat droppings run about ¾–1 inch and blunt or capsule-shaped, per Cornell IPM — far larger than mouse pellets, and usually concentrated in piles near nests or along travel paths. Burrows are distinctive to rats, especially Norway rats: ICWDM describes active burrow openings about 2–3 inches wide, clean of debris, found next to walls and buildings, along fences, and under bushes and debris, often with a hard-packed runway leading to the opening and fresh soil fanned out nearby.

Greasy rub marks — dark smears from oily fur along beams, pipes, and baseboards — and well-worn runways mark repeated travel routes, the same mechanism as mouse rub marks but heavier and more visible given a rat's size. Gnaw marks are coarse, roughly 3.5–4 mm wide for an adult rat, compared to a mouse's 1–2 mm, and rats can chew through tougher materials than mice can, including some plastics and soft metals.

Heavy nighttime scratching, scrambling, and thumps in walls or ceilings are common with rats — noticeably louder than typical mouse sounds. The single most urgent individual sign is a daytime sighting: because rats are nocturnal, Texas A&M School IPM notes seeing one in daylight typically means "a large population, scarcity of food, or illness within the population" — the colony has outgrown its ability to stay hidden.