https://www.wired.com/story/hungry-city-rats-are-looking-for-a-new-lunch-spot-near-you/

As a graduate student at Purdue University about two decades ago, Bobby Corrigan spent weekends living inside a commercial granary in Indiana. As part of his doctoral thesis, he observed how rats live, behave, and reproduce, following about 10 separate rat families on several floors of the sprawling facility. The population stood at about 150 rats, all told.

After a few months, Corrigan decided to remove their food supply by meticulously cleaning up all the loose grain.