Depends which domestics they are crossed with.
The one place I hunted as a kid had a feral white domestic duck that flew around with a flock of park ducks. We saw them most times we went hunting, but never got a crack at them. Same size or a little smaller than all the mallards.
Last time I was in downtown Naperville, there were several park duck/domestic duck hybrids that are the exact same size as the other pure mallard park ducks. They are also pretty similar in color, but they all had some white on them and not as much of the other colors as this one, but similar.
There are a million variations of just domesticated mallards and then cross them with wild mallards and you get all kinds of things.
http://www.gobirding.eu/Photos/DomesticDucks.phpMy other reason beyond the picture is his first gut reaction.
jsc85 wrote:Black duck cross? Melinated mallard?
I assume he has shot enough mallards that any large differences beyond color would have jumped out.
However, I agree. Always spread out the wing enough so it shows clear enough in the picture. I try to do that in my pictures even when I am not trying to identify ducks just so others can immediately know what is in the picture when they are not mallards.