We have several is our neighborhood that are similar to the one you
describe. They've been around for a few years and we jokingly call
them the "white-backed chickadees".

Here's a picture from a couple of years ago.

http://www.eskimo.com/~cekerns/birds/albums/bc_chickadee/bc_chickadee16.jpg


More bird pictures at http://www.eskimo.com/~cekerns/birds

Craig Kerns
Lake Forest Park, WA
cekerns at eskimo.com