A few people have asked me recently about small birds with yellow at the base of their tails, flitting around in shrubs. They're most likely Yellow-rumped Warblers, one of the most common warblers in North America. (Although it's an appropriate name, I also like one of the older names for this species, Myrtle Warbler, as it reveals some of their ecology.)
During the winter they often feed on Wax Myrtle fruit (see photos), as well as insects and other invertebrates. These photos were taken at the north end of Bodega Harbor (near Whaleship Road).