I have been trying to identify this duck for the last couple of days. Its the only image I have of the duck, I photographed it in San Diego in January. It has several distinctive features (like the bill color, underside and head color) but looking through my books I can't find an exact match. My best guess is that its some kind of Mallard hybrid. This site has a bunch of them but I don't see an exact match there either.
Technical Specs: Nikon D300s, 200-400 VR @ 310mm, f/4, 1/400s, ISO 400.