Hi! Hawks are hell as I well know fellow Ohioan. Here's some tips that worked well for me/other with a roofless pen.
1. A rooster. I don't know if this is true of drakes but a good rooster is a master of spotting hawks and getting their flock to cover. If you don't have a rooster it's possible that you could get one and your ducks could learn to respond to rooster warning calls. I lose SO many fewer birds with a rooster around.
2. Fake owls. Some people swear by them but I have had little experience with them. Rotate its location on a regular basis to make it look real. Every few days give it a new branch or tree or fence to call home.
3. Hanging CDs. Old AOL cds, blank cds, as long as they reflect light well (so no CDs like old PS1 discs that are black for example). Hanging these throughout the pen out in the open where they can sway or spin in the wind can create dancing lights the hawks hate. Rotate the locations of these as well if you can.
4. Encourage jays, crows and other aggressive non predatory birds to take roost in your trees with things like squirrel proof bird feeders and native plants and nesting materials that meet their needs. They hate hawks and will not only make a ruckus when they see one they will often harry them and drive them away. (Warning, they may choose to dive-bomb you too though!)
5. If you can't put up a full roof give them a partial top by hanging something like
flaggery/fladry in occasional locations across the pen, out of reach of nosy fowl and well within view of the hawks.
6. Make LOTS of hiding spots. You never want your birds to be more than a few feet away from good cover if you can help it. Even large scattered branches to duck under helps them get away in a pinch.
7. Harry the hawks yourself. Deliberately injuring or killing a hawk is illegal. But contrary to popular belief harassing them IS legal. You can spray them with a hose, shoot bottle rockets in their general direction, hit the branches of the trees they're in, fire old-timey cap guns, probably even plonk one with a nerf dart. In fact this is a legal REQUIREMENT for a depredation permit, so tally hoe on the aggressive but non harmful scaring away methods!
The hawks in Ohio become much more active in the fall and winter in my experience so take extra care to recheck everything in the early fall even if you've had a quiet summer. And I also find the hawks most active first thing in the morning so consider keeping the birds in an extra hour past sunrise if you can swing it.
That's about all I've got for you but I hope it helps!
