Mallards are famous for generosity with their DNA, to put it diplomatically. The males readily mate with other duck species, with or without their consent. Among the results are hybrids like this male. It has the Mallard green only on the front half of its head, and a mix of dark brown, light brown, and tan on the neck. It lacks the characteristic white ring around the base of the neck. The body feathers could be any number of ducks except for the white tailfeathers, which are again Mallard. Both the male and the female have the blue speculum with white edges (the bright patch beneath the wings toward the rear), a Mallard trait. I cannot tell what other duck species is represented here. The female could be a straight Mallard; they vary considerably in plumage.
This pair paddled west along the Virginia Street Extension until it reached the rip-rap along Marina Boulevard. There they settled down for a nice long session of preening. They showed no anxiety about humans nearby. Probably they are park-raised birds, not feral ducks that have had experience with hunters. Mallards in the wild are the most heavily hunted bird in North America, but attentive management and their own fecundity have kept populations approximately stable.
I also saw a pair of ordinary Mallards, but they were not so interesting as these odd-looking ones.
You can find other entries about our local Mallards, including other hybrids, on this blog.