Our ducks get a simple oral wormer in a syringe from the Avian Vet (administered by us at home, a few days in a row). Apparently this should happen about every 6 months to keep the worm population down.
Diatomaceous earth (food grade not pool stuff) is also sold as a general wormer. I don't know its effectiveness and have not used it yet for those purposes, but it is apparently safe to add to feed (up to 2% of the overall volume of feed). I think either way you should check with an avian vet on this one.