The brown staining evident on this individual probably comes from iron oxide dissolved in the water. Digiscoped with Panasonic DMC-LX5 | Nikon FS 3 | 30XWA | hand-held, no adapter.
Reference:
Bales. B.R. (1909). The Brown Stain Occasionally Seen on the Feathers of Wild Ducks. The Wilson Bulletin, 21(4), 221-223. Retrieved from http://www.jstor.org/stable/4154269