I'd build a pond and put their food on an island - provided you can reach the island
, or have a good cricket bowling style.
My ducks and chickens are in separate pens and sheds, so I can feed them separately, but to be honest chickens and ducks both graze all day. My hens are 'aye peckin', and my ducks will forage and snuffle about the lawn all day if I let them(they are getting rid of all the moss for me at the mo). If your ducks are leaving the pellets then they prefer what they can forage for. Can you perhaps feed the ducks inside - maybe leave their food overnight so they get it in the morning. You need to be sure you have no long tailed friends though.
(Dan, why no Rat icon?)