Those are not mallards. The drake doesn't have the right coloring, and his neck is too long. Google Khaki Campbell Ducks. It looks like that's what you have on your hands. Might explain why they haven't flown away. That, and they have found a source of food, so they have no need to leave. If no one is claiming them, I agree with the above posters that they may have been dumped. You might want to start thinking about whether or not you're going to adopt them or find them a new home.