At least the mystery is solved! Perhaps set up some netting over the yard?Caught the predators in the act: Young Red Tailed Hawks!
Note the plural!
They must have had a nest somewhere around here and now that their parents have kicked them out they are starving. I heart a commotion outside and found that one of them had just killed Sandals Duck
At least two of its siblings were lurking in the trees above the duck yard. Sunny and her ducknagers were hiding on the patio together with Harry. Tweedledee and Laura had their ducknagers under one of the raised beds. The non-runner ducks were under the duck-house and the runners hid under the tulip-tree.
Those hawks are a menace! Even with me outside, they dive bombed the duck-yard, focusing on the ducknagers. That went on the whole afternoon until about 30 minutes ago.
I have installed flashy tape, mirrors and ropes across the duck-yard as obstacles and deterrents, not sure why they have suddenly decided to leave.