Why did the tortoiseshell cross the road?

Late this afternoon, I was driving south along Highway 1 from Bodega Bay towards Valley Ford, when I noticed a fairly steady stream of butterflies crossing the road.  I thought they might be tortoiseshells, but I wasn't certain without a better view.  So I pulled over in Valley Ford to take a closer look.

The number of butterflies was impressive!  I counted more than 50 individuals in ~10-15 minutes.

They were California Tortoiseshells (Nymphalis californica), and here are two photos for the record.  Both were taken on the outer wall of what is now the Northern Light Surf Shop in Valley Ford.

 

 

Around this time last year, there was also a large movement of California Tortoiseshells.  See the post called "And the answer is..." on 26 September 2017.