Probably worth updating this post since it's been about 3 and a half months. I started free-ranging the ducks during August, probably when they were 5 or so weeks old and starting to get pretty big.
I bought a decoy owl and filled it with rocks and I move it to a couple different spots in the yard every day or so. I also have strips of scare tape dangling around a few spots in the yard which shine in the light. I also work from home, so I'm outside at random hours of day visiting the animals for a couple minutes here and there, which probably creates uncertainty for the hawks.
Anyways, haven't had any issues. I see them ALL THE TIME overhead in the farm field behind us and even directly above us sometimes. I have never witnessed them try anything and they seem relatively uninterested. No issues for basically 2 months of free ranging. They also seem to have good instincts. Whenever something flies overhead, they start looking up and will instinctively move to cover (patio chairs/table, their coop, etc).
We even had one cool experience where some pigeons flew directly overhead, maybe 15 feet above us and the ducks were in the pool at the moment and let out a loud warning quack then dipped underwater for like 3 seconds synchronously. Really cool to see their instincts in action. Anyways, there's enough random stuff in the yard for them to hide in IMO that I don't think there will be issues.
