EBird describes it as a flying ping pong ball with a long tail, and that's about right for the Bushtit (Psaltriparus minimus). They fail to mention that watching them is not unlike trying to watch the ball at pro ping pong matches. They never seem to stop moving. I've seen dozens of them on my recent walking trips in several environments, and I've wasted many minutes trying to get a reasonable picture.
I got lucky the other day on the Tuolumne River Parkway Trail. For reasons unknown to me, one of the females stopped to consider my presence and sat still for a few seconds, long enough to snap a couple of pictures. The males of these cute little birds have black eyes, but the females are quite different, with a golden ring around the pupil. To me it makes the female like the mature serious one in the bunch.