California Tortoiseshell (Nymphalis californica) is a small-medium butterfly, with a wingspan of 1.25 to 2.75 inches. It overwinters as an adult, and can be seen in our area from January to June, and then again in September and October. Locally, numbers peak in January-February. In June, it migrates to summer estivating grounds — estivation is a state of dormancy akin to hibernation, except that it takes place in summer rather than winter — in high country (generally above tree-line), and then returns in September. Technically, it is around in November and December, but is rarely sighted. Host plants are various species of Ceanothus.
California Tortoiseshell has regular population explosions, which result in huge migrations. In July-August 2004, a swarm of Tortoiseshells 40-50 miles long by 15 miles wide traveled south along the Sierra Nevadas. There was a smaller population explosion in 2019.
