Black-bellied WD are most certainly expanding their range. Memphis area had over 50 breeding pairs last year, everyone along the Mississippi River and its tribs stand a reasonable chance of seeing them in the coming years. I know they wonder around a lot in spring, perhaps looking for new breeding areas (they see them up the East Coast in small flocks). I know there have been single individuals to small flocks scattered throughout the Midwest in fall, so likely the young birds may do kind of a northward dispersal in fall (and they can show up at random bodies of water). So keep your eyes out during season, they may turn up in your area.
For those of you who hunt them, do they decoy to "traditional" duck decoys or do you have to almost strictly target whistlers (Fulvous and BB)?