Perhaps, after you video tape him, discreetly, you can ask him if the ducks are bothering him. Or, you can just straight out ask why he does that. I know with me, I would love to "talk" with my neighbor's ducks if I lived in a single family home. I talk to the ones as the lake (but with human sounds not quacking at them).
For me, wondering why he was doing that or if he was the one who complained would pick at my mind. You don't have to ask if he's the one who complained, but you can probably find out if he's annoyed by your ducks by talking to him about them a little.
ETA: Perhaps, if he's doing it because he likes them, you might have an ally on your side.