We have had a "pair" of Mallard ducks, come to our pond every year for
the past 5 years. We are in Central OH.
They come like clockwork in late April, stay for approx. one week-- then
leave for their permanant summer home further north of us.

Aside from occasional duck droppings on my stones, which hose right off,
they do no harm. In fact, I think any droppings that may go into the
pond, jump start the plants.
While I would never introduce any ducks to my pond on purpose, these
two, are harbringers of Spring and a most welcome sight.