The Western Tiger Swallowtail butterfly (Papilio rutulus), seen here, is a native of western North America, from the Rockies (and sometimes farther east) to the Pacific and from British Columbia to Baja California. Coastal Northern California, where I live, is one of its favorite habitats. It is a large butterfly, with a wingspan of about three to four inches (females are larger than males). It can be seen flitting about from spring through fall, and occasionally even in winter.
Art Shapiro, professor of Evolution and Ecology at the University of California, Davis, says that “The Western Tiger Swallowtail is basically a species of riparian forest, where it glides majestically back and forth along the watercourse. It has expanded into older urban neighborhoods where several of its host genera are grown as shade trees, and behaves as if the street were a watercourse. In the high country and on the Sierran east slope its usual host is Aspen.”