Norway Rats: Why Chicago Has So Many
Chicago has topped the rattiest-city list for years. Here is the rat behind it and the city ecology that feeds it.
Meet Chicago's rat
The rat you have in Chicago is almost certainly the Norway rat, also called the brown rat or sewer rat. It is a stocky, ground-dwelling rodent with a blunt nose, small ears and a tail shorter than its body, and it can weigh close to a pound. Unlike the climbing roof rat that dominates warmer coastal cities, the Norway rat is a burrower. It digs nests in soft soil, prefers to live at ground level, swims well and readily uses sewers and basements. That burrowing behavior is the single most important thing to understand about controlling rats in Chicago.
Why Chicago tops the list
Chicago has been ranked the rattiest city in the United States for years running, and the reasons are structural. The city has roughly 1,900 miles of alleys, more than almost anywhere, and those alleys are lined with garbage carts that feed rats year-round. Dense housing packs buildings close together with narrow gangways between them, wooden back porches sit over soft burrow-friendly soil, and a huge restaurant scene generates constant food waste. Add ongoing construction that disturbs and scatters colonies, and you have close to a perfect environment for Norway rats.
How they live on a city lot
On a typical Chicago lot, the Norway rat colony lives outside and commutes in. Rats burrow in the soft ground under the back porch, deck or shed, and along the foundation and gangway. They travel the alley at night, feeding from garbage carts and dumpsters, and they use the sewer lateral and gaps where utilities enter to get into the basement. From the basement they move up through wall voids into the living space. Understanding this route, alley to burrow to basement to house, is what makes control work, because it shows why treating only the inside fails.
Winter drives them indoors
Chicago's winters change rat behavior. When the ground freezes and outdoor food gets scarce, the colony pushes indoors toward heat and shelter, which is why rat calls spike from late fall through winter across the North Side. Rats that spent summer in a back-porch burrow move into the basement, the walls and the garage. This seasonal push is a big reason exclusion matters so much here: sealing the building before the cold arrives keeps the outdoor colony from moving in when it gets desperate.
What actually controls them
Because the Norway rat colony lives outside and burrows in, control has to treat the whole lot: find and treat the burrows, trap and remove the colony, and seal the building so the constant outdoor pressure cannot restock it. Poison alone does not work, it leaves the burrows and routes open and creates dead-rat odor problems. The proven sequence is trap, remove, seal and clean, adapted to the Chicago lot with its porch, gangway, garage and alley. For the details, see how to get rid of rats and Chicago rat extermination.
Sources and further reading: www.cdc.gov, en.wikipedia.org.
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