This is the White-faced Whistling Duck, Dendrocygna viduata, a quite noisy duck that breeds in sub-Saharan Africa and also in much of South America. One hypothesis is that, at some time in the past, humans transported some birds to a new, far flung location.
The white-faced whistling duck has a grey bill, black neck and head and a white face. However, there are regional variations, such as these I encountered in the Kartong wetlands in The Gambia, where the face is more brown than white.
The back and wings are dark brown and the underparts are black with white barring on the flanks. Both sexes have similar plumage.