What about a 2 legged predator? Moved the cage away from the house a ways, in case they quacked noisily while they got them out? Didn't you say they took the ducks you wanted most, and left one little off-breed?
Probably a raccoon. They can reach thru wire and pull them out a piece at a time. Ducks wouldn't have feathers yet, so there wouldn't be much to see left. What size wire did your cage have on it?
Sure sorry. That is so disappointing for you, I know.