Well, the ducks were stronger than we accounted for, and they were able to push through the barrage we had made. I went to close up the hens a little before sundown and found that the ducks had literally flown the coop.* I spent almost 2 hours looking for them in the rain and dark, to no avail. I went out yesterday, checking all the nearby ponds, again with no luck. I think they are well and truly gone. I'm so bummed, and it isn't just for the financial loss. We have so many foxes around here; I don't know if the ducks can make it out there on their own.
It is a long shot, but if the farmer we bought them from is at the next market, I'll ask him if they flew home. I don't know where he lives, but I understood from what he said that his farm is minimum 40km from here.
* It didn't even cross my mind to ask the farmer if he had clipped their wings, and while I don't know for sure if they flew, I could find no trace of them around the house.