This bird was seen on 1/3/2011 in the San Francisco bay area. It was with a group of 4 that looked the same, two of which had gray (slightly darker thank wings) on the top of the head (instead of white). I have never seen them here before, or since.
It's a Forster's Tern. The first thing to note is that this is a small, dainty bird with a relatively small bill. This eliminates the big terms like Caspian, Royal, and Elegant. Next, note the dark eye patch but no dark cap. This is classic Forster's. Finally, look at the primary coloration. They are whitish, not dark. This too is classic Forster's. Common Term would show a dark cap and dark contrasting primaries.
Thanks Paul. I have been fooled once again by winter plumage!! There are tons of Forster's around here, but I guess (until now) I have never seen one in winter colors. Thanks for the quick reply...