Two years ago I noticed a dark brown duck with a mottled whitish front to its neck hanging out with the local mallards on the river in downtown Northfield. It was suggested to me that it might be a domestic Swedish duck or a cross with one (here's a photo from another source, FowlBlog.com, that looks quite similar). Last summer I saw it again, or another very like it.
A couple of days ago I saw two of them, snoozing on the eastern bank near a lot of mallards (above). They were very sleepy, so I didn't get a shot of one with its head up, but you get the general idea. If you click on the photo, you'll be able to see it much, much larger.
Here are some of the other ducks that were in the vicinity. It was nap time for most.