I have been reading that the following plants are good to plant in a duck pen.
Ornamental grasses, including pampas and ferns,
Evergreen plants such as:
Pieris ‘Forest Flame’,
Small ground cover plants such as:
The source for this info was http://www.waterfowl.org.uk/docs/plants_and_pens.pdf
I am sure I have read somewhere that lily of the valley is poisonous to ducks but can’t remember where I read this.