In between appointments in Santa Rosa today, I caught a glimpse of a hummingbird flying towards a tree and then disappearing. At first I wasn't sure where it went, but then I realized something was going on. The hummingbird wasn't sitting on an exposed branch, but it was hunkered down in a small fork, where it was building a nest!
She kept flying away and returning with different nest materials. Above, perhaps some fluff from a willow? Below, a piece of moss: