I was laying in wait for a nesting House Finch pair to show itself when this big bright yellow butterfly fluttered by. By pure luck it halted for just a moment on the flower of a wild radish and I was able to snap a photo to record its presence here. I saw several others at different points in the park in the days following. This is an Anise Swallowtail (Papilio zelicaon), a common enough butterfly of the West Coast, but not always prominent in the park. I last photographed one (and better) in 2016, here.
As the name implies, it feeds on fennel, of which there is a generous supply in the park. It also likes poison hemlock, ditto. It has an interesting defense mechanism. If disturbed, bright orange stinkhorns suddenly jut out from behind its head, and these emit a foul odor.