Well... I think chickens will eat any kind of grain, and household waste too, so maybe that goes for ducks too?

[Look, they're the same basic shape, so that should work, right? Help this kid, the only experience he's ever had at taking care of an animal was a pig, and he doesn't even remember that in so many words. He just remembers the general facts more than the specific details of his own life.

...But oh, look! His eyes widen, and the ducks are suddenly definitely surging forward toward Sanae.] Ah, that's amazing! Is that your power as a Goddess? [It kind of has to be, right?]