Thank you all! Yes, I realize that if I let them have the run of the yard, some will get eaten by other animals. I just lost a hen to (I think) a hawk. And while I was very sad, I knew that this was the possible price of their freedom. Since then, I've been valuing their lives a little more and their freedom a little less, but it's definitely a balancing act. It's easy to justify right now. I only have two hens (can't afford a loss for the sake of the survivor), and I need them doing the spring tilling, so they are confined to a 5x5' mobile pen whenever I let them out of their enclosed yard.
But with ducks...giving them access to the water eliminates the utility of a mobile pen. I could set up a fence, but given where the water is, I doubt I could set up an electric fence without doing some brush and sapling removal first. And neither a physical not electric fence is going to stop hawks anyway. It's much to think about. I'm hoping that adequate numbers kept together with a physical fence may be something of a deterrent to predation. And if I have to occasionally tithe to the wildlife, I can be OK with that so long as the flock can absorb the loss. After all, I moved here only about 3 years ago. The fox and the hawks have been on this land for generations.
I guess we all just have to find a way to keep our birds safe without making ourselves crazy nor inhibiting the natural behaviors of the flock.
But it's good to know they aren't likely to just wander off.