Piebaldism.

Most people will have seen that ultimate contradiction in terms, a white blackbird. Apparently blackbirds are particularly susceptible to faults of this kind.

But here we see what I believe to be a largely white Mallard. At least it was happily swimming about with a large group of ordinary Mallards.

Had a quick look at Wickipedia from which it appears that this is a very complex subject involving albinism and about which there is disagreement among scientists.