I looked at Merk's Veterinary Manual and it said nothing about worming ducks.
I know you do not want to use a wormer that has arsenic in it with ducks or geese.
Piperazine is a common wormer used on chickens, turkeys and pigs for Nematode or roundworms. The Food and Drug Administration has approved the use of piperazine only in swine, chickens and turkeys.
Below is an article published by the Food and Drug Administration. They mention dogs, cats, chickens, turkeys, horses, swine, sheep, and cattle. No ducks. http://www.fda.gov/ohrms/dockets/98fr/042999a.txt
All piperazine does is paralyze the worm so the paralyzed worm leaves the body of the host with the feces.
If I had ducks, I would go ahead and worm them with piperazine. I read a study in which the ducks were given piperazine and the ducks did not suffer any ill effects from the treatment.
I would follow the label directions for chickens.