I had the unique pleasure of seeing this live last weekend while at a friend's home for dinner, and sure enough, some other Bay Area native was smart enough to get it on YouTube.
This video doesn't really give you the sense of all these thousands of birds swooping and swarming together in this dance. You can see the individual birds, but they move like a single, undulating entity. It is truly hypnotic to watch. At sunset every day in Marin County, California, residents gather to watch this wonderful sight.