I'm used to seeing Gulf Fritillaries (Agraulis vanillae) in Florida and other parts of the southeastern U.S. (The "Gulf" in their name refers to the Gulf of Mexico.) I was surprised at first to learn that they occur in California.
This is a tropical/subtropical species, occurring from the southern U.S. through Mexico, the West Indies, and South America. Gulf Fritillary caterpillars feed on Passionflower vines (Passiflora spp.), none of which are native to California. However, where the plants have been used in landscaping, the butterflies may follow, as long as temperatures allow (they're apparently sensitive to temperatures below ~22°F).
Note that Gulf Fritillaries are not "true fritillaries" — that is, they are not related to butterflies in the genus Speyeria with which they share a common name. Gulf Fritillaries are in the subfamily of butterflies sometimes known as "longwings." It's hard to see this feature in my pictures, but if you encounter this species in person, look for the very long, narrow forewings.
Although I cropped the pictures above so you could see the butterfly up close, this butterfly was very well camouflaged while resting high in the branches of this shrub. Can you find it in the picture below?
The Gulf Fritillary is just below and right of center.
To learn more about the history of Gulf Fritillaries in California, read Art Shapiro's summary here.