Well, I never would have guessed, but a source on the ‘Bird ID Help’ group on Flickr helped with this one:
I was thinking it was an immature red winged blackbird, or maybe a young cowbird, since we had just seen those.
This guy doesn’t have the classic bobolink plumage on his head yet. Bobolinks go through two molts every year, and this one is just coming into his breeding plumage. Hopefully, he’ll be back in full regalia for another picture.