Oddly enough, there is a remote possibility you are dead right here. The sole flock of wild whooping cranes migrates from Aransas NWR, Texas to Wood Buffalo NP, Alberta, along a flight path that is significantly west of the Wisconsin morainelands, essentially the western Dakotas and adjacent eastern Montana at that latitude. However, this site describes the new whooping crane reintroduction project which is supposed to introduce a second flock which would winter in Florida and breed in Wisconsin. I’d rejected looking up the whoopers because I knew Wisconsin was too far east for their migratory flight; I didn’t know about the introduction project until I looked up what the migration area for the wild flock was.
So QtM, you may have hit the jackpot: a sighting of a wild whooping crane! My hunch, though, is that it was a sandhill.