Of my 4 dogs:
Pointer: pretty much ignores them. In a large area I trust him with them, but I'd never keep them locked up together in a small yard, because he might get bored and decide to chase them for entertainment
Lab: not at all interested in the ducks, but they chase him and he gets annoyed. That's the only way I can ever see him hurting them is if he got fed up with them and snapped at them
Golden Retriever: scared of the ducks, when they chase her she lays down and looks at me like she saying "help!" They climb on top of her, biting her, and she'll lay there until I call her away, at which point she jumps up and makes a beeline for me, who will protect her from those scary ducks
Large breed mutt: would kill all ducks, chickens, cats, rabbits, snakes, birds, rodents, etc. etc. pretty much anything that is not canine is fair game