Thank you everyone for your suggestions about my mystery bird.
Background: My property in Everett is a Backyard Wildlife Sanctuary on approximately one acre. I am learning birds by observing several feeders near the house and thumbing through a stack of bird guides. I was convinced that the bird I posted my question about was neither a white crowned sparrow nor a mountain chickadee. I asked about the great tit because it is listed in Peterson's Guide to Western Birds. Based on the replies I have received, I now feel that it was a foolish suggestion, and I apologize for wasting anyone's time. I will continue to observe this bird, and I do not feel it is worth anyone's time to come here to photograph it.
Thank you again for contributing to my education about local birds.