I agree with
@chickens really. Ducks need a complete feed. If they're free ranging, they might not need as much, but keep it available anyway. Proper nutrition is important, no matter what type of poultry you raise, but an inadequate diet will cause serious problems for ducks in particular. There's just less room for error with waterfowl and game birds. Over time, nutritional deficiencies can lead to death, or contribute to death via weakened immune systems. Fresh water daily, deep enough for them to dunk their heads, is also a must.
All this being said, we can't really give you a definite cause for the death. It may have been completely unrelated to diet. However, as a general rule, healthy birds are less likely to die from unknown causes, so I'd still recommend improving their diet.
