There is usually a gang of about 50 or so whistling ducks hanging around in Terry Hershey Park when we visit Houston. They are quite notable for their whistling habit.
This particular variant is called the black bellied whistling duck, for obvious reasons, found all year round in South East Texas.
Wikipedia lists 8 variants of whistling duck, or tree duck, and suggests that they are not actually true ducks but “a subfamily, Dendrocygninae, of the duck, goose and swan family”. They do look sort of intermediate between duck and goose.