Excerpted from Butterflies of the Bay Area and (Slightly) Beyond: An Illustrated Guide, coming September 30, 2025.

drawing of a tortoiseshell butterfly
California tortoiseshell, dorsal. Its host plants are an array of California lilac (Ceanothus spp.). (Illustration by Liam O’Brien)

Butterfly Migration

I remember living in the Richmond District of San Francisco along Golden Gate Park for a while, and there was a particular day when I was walking and noticed a butterfly flying above me. As I watched it, I saw another, and another, all blasting down Fulton Street. They seemed to be coming out of the park, then heading due west up the road. Another and another, all just going over the tops of cars and my own head. I looked around madly to connect with another human and to verify what I was seeing. It was a California tortoiseshell (Nymphalis californica) “movement”—an explosion of them popping out of their chrysalises after a few days of extended heat, and I was caught up in it, awestruck. 

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Liam O’Brien is a self-taught lepidopterist and illustrator. He used to be a professional actor, having appeared in Les Misérables on Broadway, but shifted his powers of observation towards nature several decades back. He’s fascinated not only by butterflies but also by our relationships to them. He surveyed the county of San Francisco, where he lives, for which butterfly species remained in 2007 and 2009. He is the creator of the Green Hairstreak Project for the organization Nature in the City, and he led efforts to restore Variable Checkerspots to the Presidio. Since 2015 he has helped monitor the endangered Mission Blue butterfly in the Marin Headlands. O’Brien was the recipient of Bay Nature magazine’s Local Hero Award for Environmental Education in 2014. He lives in San Francisco.