I do hope your duck recovers. She will be in shock , and who can blame her. Hopefully, now its dark, and things are quiet she will calm down. I don't suppose you will know if the fox has caused her any damage. Their bite can carry all sorts of nasty germs and if there is broken skin it will need bathing. I have had a few hens picked up by young foxes, and after a few yells from us, it has dropped them, and each time we have been lucky that although shocked, they have recovered.