This is a photograph of an intersex Mallard. In this case, we have a female duck which is also displaying some secondary gender traits of a male Mallard—most notably in its plumage coloration.
This unusual condition occurs for a couple of possible reasons. One might be that the duck has the gonadal tissue of both male and female ducks.
Another possibility is that this is simply a very old female duck and that her natural production of female hormones has declined as a result of her age. This occurs occasionally at places that are especially conducive to health and well being of the birds. Places like the duck pond where this bird was photographed.