Calcium in large quantities can be just as bad as too much of anything else. It seems be the one thing though, i see people overdosing their birds on. Anything in moderation including spinach is fine. Calcium can block the intake of iron and our birds do need iron, they need a healthy balance though.
We actually have a bird here who is anemic and part of the problem was the layer feed, the oyster shell and some of the other calcium rich food hindered her iron absorption. I add spinach to her gavage mix to bring up her iron and switched her off layer feed. In her case the anemia is a secondary problem but was aggravated by the calcium push in our feeds. So it goes both ways. Give your ducks what they like but as a treat only and in moderation. Overdoing one food is never a good thing.
I have seen several cases of people feeding so much calcium to their ducks that the ducks body stops producing the calcium themselves and your pretty much stuck giving them a supplement for life, and it damages the bones you just don't realize it.
We get a large volume of produce donated to us from the grocery stores. We run all the harder foods through the processor on shredder mode and everything else gets chopped. We have a mucking trench we pour it into and its like Christmas. We put everything out and they will take what they want and leave what they don't. The trench gets flushed out every day and the leftovers leave. We give them bags of spinach and lettuce but they don't get much of one thing just a large variety. Its enough to maybe replace one meal for them. They rest of the time they are eating pellets or mucking for bugs. By and large the favorite is the tomatoes. Clarence begs for them.