Something ate the head off my female mandarin. My guess is a skunk. I've taken steps to further secure the pen, and am wondering if anyone in Michigan has a female they would be willing to sell.
Someone earlier asked about lining their pen with stones. My pen is lined with river rock and the advantage is you can wash the poop right down under the stones so it stays much cleaner. The disadvantage if you live in cold climates is the "tongue on a flagpole" effect that they have. They tend to stick to the duck's bodies.

Fortunately the ducks learned quickly that getting into the pool causes them to fall off. Every week when I rinse out the pool I find several stones in the water, and the pool is raised a couple inches above the stones so they couldn't just fall in.