The Norway rat, also known as the brown rat, isn’t native to California. NatureServe lists it as exotic here, and its presence traces to ports, farms, and city blocks rather than any natural range expansion.
Records stay close to flat through the year, running between about 55 and 90 a month with no real spring or fall surge. That steadiness fits an animal that lives wherever people do, tied to buildings and food waste rather than a wild breeding season.
Reports split almost evenly between iNaturalist photos (671) and museum and survey records (688), a mix that makes sense for a species both easy to spot around cities and regularly caught in pest surveys. Its statewide record count puts it among the more frequently documented mammals, which mostly says how often people run into it in developed areas, not that California is especially good habitat for it.
Full species profile
