A pine siskin:
Except for the heavily streaked breast, it's very easy to mistake them for goldfinches--they're about the same size and the pattern on the wing is similar.
Siskins are pretty common, we see a few every year--but last year there was an irruption of pine siskins and common redpolls. We were inundated with both species! Very exciting!
Redpolls (notice the red on the head that helps distinguish them from pine siskins and goldfinches).
We probably won't see another redpoll for 2 or 3 years still the next irruption.
And here are some American goldfinches in winter plumage at the feeders for comparison.
All three of these species are similar in size and often feed together at the feeders.