I found this unusual duck yesterday in my local patch, Donaldson Park. It is almost entirely white, except for random speckling on the head and back, the dark flight feathers, and the tail and tail coverts. I think this is a leucistic female Mallard. Leucism is a condition in which an animal loses some or all of its pigmentation so that it appears pale, all white, or with white blotches around patches of normal color. Unlike albinos, which lose all of their melanin, leucistic animals typically retain their normal eye color.
With Mallards, there is always a question of whether an unusual individual is a wild Mallard, a hybrid of a Mallard and some other duck, or a domestic Mallard breed. I think this is a wild Mallard for a few reasons. First, the bird seems slightly smaller and more compact than the normal wild female and male Mallards close to it, but not so small that it would have required selective breeding to produce it. Second, its bill is similar in size, proportions, and color pattern to the nearby female's.
Third, although the wing feathers on the two birds appear to be folded slightly differently, the overall color patterns of the plumage appear similar on the white duck and the normal female Mallard. The head of the white duck has fine, dark streaking; the breast, back, and tail coverts have teardrop-shaped dark streaking; the tail and tertials are dark; and the speculum (barely visible in these photos) is blue with black and white borders. The blue on the speculum is only visible in the photo with the wild male Mallard, and even then it is only a small part of it. I do not see strong evidence of another species's plumage pattern in this bird.
Regardless of its true identity, this is an interesting and beautiful bird. If you happen to see reasons why this is not a leucistic wild female Mallard, please leave a comment!