When we lived out in the country we looked out one day to notice 2 rats under the birdfeeder. They fled under the well house when we opened the door. On further exploration, there was an entire colony of rats that had dug a burrow under the pump housing. I won't recount the gruesome details of what we went through to eliminate what eventually turned into more than 60 rats, but during the time we were trying to eliminate them, some of them made their way into the storage compartments of the Airstream we were living in. It took months to get rid of them. Since that time, I can not abide rats and assume that if there is one, there will be more. Norway rats live in large family groups.