Sometimes I think it’s best to let nature run it’s course.
Let’s say you managed to scare the heron, but…..
G-d forbid if you did interfere, and the baby duck ended up with a broken limb and ended up in a hole and starved to death slowly, or other such scenario. When animals are hungry they kill quickly, and that is the way it should be.
And even if you did manage to get the baby duck to a vet. Are you prepared to pay the vet bill for an animal that will only let be let out into the wild?