I am somewhat loath to post this, due to the rareness of the bird in CA and the difficult place it was seen in, but today, 4/9/11 @ 2:30 pm on Twin Ponds Trail in the Walnut Creek's Shell Ridge Open Space (Contra Costa Co.), I observed a Cerulean Warbler. I had a good, if short view of the bird, before it entered a thicket and did not come out in the time I had available. Access is about a mile in from the Whitecliff Way trail head, but I would discourage trying to find it unless you simply want a good walk. If you do, take the left path up the hill; turn left at the top, taking the very small path down into the adjacent valley. When you meet the cross trail, continue straight on the increasing deer trail-like trail, and continue up the canyon. You will eventually cross the little canyon stream, and shortly dead end into a more formal trail, with a wooden footbridge immediately to the left. The bird was in the thickets and oaks immediate to the right (you will see blooming Bower Brush).
Anthony Cermak, Sonoma Valley