Dark-eyed Junco

from allaboutbirds.org :
Dark-eyed Juncos are neat, even flashy little sparrows that flit about forest floors of the western mountains and Canada, then flood the rest of North America for winter. They’re easy to recognize by their crisp (though extremely variable) markings and the bright white tail feathers they habitually flash in flight. One of the most abundant forest birds of North America, you’ll see juncos on woodland walks as well as in flocks at your feeders or on the ground beneath them.

Oregon Junco (from wikipedia):

'These have a blackish-gray head and breast with a brown back and wings and reddish flanks, tending toward duller and paler plumage in the inland and southern parts of its range[5]. This is the most common form in the west, found in the Pacific coast mountains from southeastern Alaska to extreme northern Baja California, wintering to the Great Plains and northern Sonora. There is an unresolved debate whether this large and distinct group is not better treated as a full species.'