Dark-eyed Junco

Lots of these around - they tend to be in groups and feed from the seeds that are dropped from the feeder. It took a while to figure out what they were as their coloration isn't anything special. However when they fly, or hop on the ground, their white tail feathers flash - none of the guides except for Wikipedia think to mention this most distinctive feature. That said, I still edited Wikipedia to include the "hopping on ground bit".