It's getting cold at night and those ducks must look awfully tempting. We lost the last lot of ducks to a fox on a particularly cold night - Charlie had ignored them up til then.
I've set up one of the excellent Minox DTC600 trail cameras to watch the pen so I can find out more. For the technically minded, that's a pigeon hide pole, with a Manfrotto 143BKT bracket which just happens to fit perfectly - it's a super easy way to place a trail camera anywhere you can drive in the spike, and adjust the height and angle by loosening a single screw.