California Tortoiseshell (Nymphalis californica) is a small-medium butterfly, with a wingspan of 1.25 to 2.75 inches. It can be seen in our area from January to June, and then again in September and October. Locally, numbers peak in January-February. It breeds in March/April (with adults emerging May/June).  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. It overwinters as a hibernating adult in October, emerging in January. Host plants are various species of Ceanothus.

Adults live for 9 or 10 months, making this one of the longer-lived butterflies in North America. However, they are inactive for roughly half that lifespan due to estivation and hibernation. Only two of our local butterflies both estivate and hibernate over their life cycle. The other one is Mourning Cloak, where once again it is adults that engage in estivation and hibernation.

California Tortoiseshell has population explosions from time to time, 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.

It is found primarily in the western half of North America, from British Columbia in the north down to Baja California and New Mexico in the south. However, there are stray sightings as far east as Toronto and Pennsylvania.