I've already ordered one, on the basis we could limit the amount of food the ducks get from it by how we use it
So the plan is that it lives in the hen's house run, and we'll put enough feed in it when we shut them up at night to give them as much as we think they'll eat in the morning before they're let out. If they don't eat every last pellet, and the ducks do find it and how to use it, the ducks won't get a pile of feed so it won't be a big problem.
During the two week training period, when it's effectively open all the time, we will endeavour to only put enough in it that it will definitely be empty by the time the choox are let out, so the ducks won't also get trained during this time