I USED to let two ducks forage all over our several acres. I had two losses due to hawks. When a hawk got my beloved Khaki Campbell drake, it broke my heart. I got a Welsh Harlequin drake to keep our Welsh Harlequin duck company and now I ONLY let them out to roam if I am outside with them working. I will go inside and keep checking on them, but never let them out ALL day anymore. We have our large duck pen completely covered (even the top) with wire and there are three strings of electric fencing, and a duck box we lock them in at night.
What I am trying to say, it is great ducks can forage outside the confines of their duck pen (mine forage and are out EVERY day-even in cold weather) but they can be easy targets for hawks and other predators.