That's it!. I give up!
This morning Mrs Duck was on the lawn with her two male admirers, and she was behaving in a way that in a human female would be described as coquettish.
I took the opportunity to check her nest. It was empty, the lining slightly dischevelled but no obvious signs of predation.
I can now say categorically, with unshakeable conviction and without fear of contradiction, that I have absolutely no bloody idea what is going on.
But I have made a resolution here. I am going back to my old ways, that is, thinking of ducks only in the context of canard à l'orange, ànec amb figues and that cute way the Chinese have of making the outside all crispy.