hiya again
mealworms are a nice treat!
the general rule for best health is only 10% of their daily diet comes from treats
the other 90% a good waterfowl feed like Mazuri or a duck feed or an all flock feed.
and avoid crumbles, pellets are better for ducks
oh also i just noticed you said “ continual access to, waterfowl feed, and scratch grains” ... cut out the scratch grains that’s very poor nutrition perhaps your ducks are eating too much of that, and not getting an adequate amount of protein and other nutrients they need from their waterfowl feed