Here is a snap shot of what appears to be a Mallard x Northern Pintail hybrid. (Note that the bird is back-lit in this photo.)
The bill had a central dark area on top, somewhat like a Northern Pintail. The head color looked part metallic green (Mallard-like) and part blackish, depending on the viewing angle. The breast is tan / light-brown in color. The white neck-stripe is somewhat like that of a Pintail. The tail is longish but curls upward as seen here.
Here is another view (Click here)
Location: Shadow Cliffs Regional Recreation Area, Pleasanton CA
Date: January 23, 2005