Could you post a video of the duck? To post a video, upload it to a video platform like Youtube or Vimeo, and copy/paste the link here.
What is the temperature where you live? What are they being fed? Is there any chance the duck may have eaten something toxic? Have you inspected the duck's legs over for any swelling, discoloration, lesions, etc?
There are lots of conditions that may cause similar symptoms to what you're seeing. Hopefully, by providing some more information about the duck, we can further narrow down what might be the problem. For now, it may be best to move the duck inside into a quiet, dimly lit area, with feed, and water provided.