When Don and I went on our big lunch date a couple weeks ago, we observed something interesting while we were getting ready to leave the small mountain town where we had lunch. Don was driving, and he pulled into a parking lot so we could switch drivers.

"Look, a dust devil," I noted, pointing at a few hundred leaves caught up in a circular dance in the parking lot.

But a closer look revealed it wasn't a dust devil. Those leaves were butterflies. Hundreds and hundreds of California tortoiseshell butterflies swirling in a circular formation.

All of a sudden, we saw these butterflies everywhere on the drive home. Why all of a sudden? We speculated the day's temperature had warmed up just enough to make them active.

We're seeing a lot more California tortoiseshells here at home too, even this late in September.

California tortoiseshells are kind of fun. With their wings upright, they're a dull brown-black, which resembles bark...

...but in the open position, they have a bright pop of orange and black color.

Apparently the California tortoiseshell is restricted to laying its eggs on various Ceanothus species (snowbrush, deer brush, wild lilac, etc.), but I have not observed any Ceanothus plants in our area. (They're easily identified by the three large midveins in the leaves – various Ceanothus species were common in Oregon where we used to live.)

However the tortoiseshell is also known for periodic population explosions (termed "irruptions") which may send them into areas far outside their normal breeding range, so perhaps that's what we're seeing at the moment. We may also be in the midst of a migration.

Whatever the reason, they're pretty creatures.