A brand new bird species in Massachusetts!
I thought, at first, that I had finally caught sight of one of the rare geese in the area. This bird wasn't in any of my field guides, though, so I knew it was either a strangely mutated Canada Goose (which seemed unlikely considering the shape and size of the bill, which looked like a whole different species of goose), or a Canada Goose hybrid. It turns out to be a hybrid of a Canada Goose and some other goose.
The pictures are horrible, so you'll have to take my word for it when I say it looks very much like the head of a Snow Goose attached to a Canada Goose's body. A little bit disturbing, actually! I found out that geese do hybridize quite a bit, and I even found a picture of one like this one: Bill Schmoker's possible Canada x Snow Goose
Jim Berry, from the massbird email list, said that "the size of this goose (same as the Canadas) implies that it's a hybrid with a white domestic goose". He could very well be right.
The black bill and small head has me thinking Snow Goose X Canada Goose hybrid still, but you know me - I always go straight for the exotic if at all possible!