Rats constantly leave droppings in areas they frequent. Fresh droppings are dark in color and soft in texture. After three days, they harden and lose their dark color.
Rats urinate in areas they frequent. Since urine gives off a fluorescent glow under ultraviolet light, a blacklight can be a useful tool for locating areas of rat contamination. (Remember, though, that other materials also fluoresce and can cause confusion.)
Rats always travel the same runways and leave "smudge marks" - a buildup of dirt and oil from their fur - along walls, pipes, gnawed openings and particularly beams and rafters for roof rats.
Rats keep indoor runways, or well-used paths, free of cobwebs, debris, and dust. Norway rats' runways are usually well-defined paths at floor level next to walls and other vertical surfaces. Roof rats' runways are above on rafters, pipes, etc.
Outside, roof and Norway rats' runways appear as narrow paths through vegetation.
Rats make sounds when climbing, gnawing, clawing and moving.
Footprints and tail drags can be seen in dusty locations. Nontoxic tracking dust such as talcum powder or flour is used to determine if rodents are frequenting certain areas.
Gnaw marks are a sure sign of rats. On wood, the older the gnawing, the darker the wood.
Rats produce a distinctive musky odor. An experienced rodent-control person can tell the difference between mouse and rat odors.
Nests are another sign. Norway rats usually burrow, but nests under concrete slabs, in rag piles or in lumber piles are not unusual. Roof rats' nests are usually up high and are often difficult to find. Sometimes nests are similar to tree squirrels' nests, consisting of leaves, twigs and vines.
After finding signs the population is estimated. A good flashlight as well as the need to get down on hands and knees is necessary.
Light Infestation (less than 20): A few fresh droppings found in out-of-the-way areas. Fewer than 30 droppings can be found in any one place, and the number of dropping locations is less than six.
Medium Infestation (20 to 50): Fresh droppings seen in 20 to 30 locations. Old droppings and fresh gnawings found. One or more rats seen at night; no rats observed during the daytime.
Heavy Infestation (greater than 50): Fresh droppings seen in numerous locations along with old droppings of various sizes, indicating an increasing population. Fresh tracks, gnawing, smudge marks and three or more rats noticed at night, or rats seen in the daytime.
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"Telltail" Signs of Rats