They do leave food behind, but no preditors ever eat the food. I have had corn spoil or rot. I think my dogs keep all preditors at bay. The ducks sleep in the cat tails pretty thick. For the winter I am building them a floating house with aeration to prevent freezing of the water around their houses
I just don't know then, hard to understand somethings with our ducks.. this maybe one of them. I suspect once real cold weather sets in your going to find them hungry. So bring out the food then, it maybe they are getting enough to eat now. Sounds like a real nice plan be sure to post pics when finished it will give others ideas.