Wild Kingdom

We hung a bird feeder on the fence in our backyard, and while squirrels ate most of the apparently delicious nutty suet - mmm... nutty suet... - on one particular morning, we had about four different birds taking their turns going for the suet dregs, including a sparrow, some kind of finch (we think), and these two in the picture below:
Obviously, the red one is a cardinal, but we're stumped as to the species of the black and white bird. Is it a woodpecker? Anyone know? If you can identify the mystery bird, please leave a comment and let us know what it is, because we've been wondering. Seriously, it's keeping me awake at night.

Thanks!

-EB
(Clearly not Marlon Perkins. Or that Audobon guy.)

UPDATE! 1/8/08, 2:40pm
Thanks to Bart Hoffstein for identifying our mystery bird as a female Downy Woodpecker. Bart even included a link: http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Downy_Woodpecker.html