Well then if you want the ducks to have all their "lots of space" then you will just have to do all that and hope for the best. Or, you could fence off an area and contain them to where it's more realistic to keep a more careful watch over them instead of having them roam all over the place.
You better button up the rest of your flock because that bird will be back and probably really soon! It was probably either a red-tail or red-shouldered hawk