Bodega Bay is close to the northernmost site for overwintering Monarch butterflies on the West Coast (it looks like Rockport, CA, in Mendocino County is the limit). Sites to the south (e.g., Santa Cruz and Pacific Grove) support much higher numbers, but up to 5000 overwintering Monarchs have been counted in recent years in Bodega Bay.
In California, Monarchs roost primarily in eucalyptus, Monterey Cypress, and Monterey Pines. I don't know a lot about the local timing, but it sounds like they arrive at the overwintering sites in mid-late fall and remain until about mid-March.
On warm, sunny days in winter on Bodega Head, an occasional Monarch may be seen gliding by. On 18 February 2012, it was very windy, but fairly mild (~51°F or higher in the sun), so I decided to see if I could find more butterflies in wind-protected areas of the Bodega Dunes campground.
There was a minimum of 25 individuals basking high in the trees — beautiful orange against the greens of the trees and the blue sky.
Above, note the center butterfly is a male, with expanded oval black spots in the middle of the inner vein of the hindwing.
Soon these Monarchs will be on their way inland and north!