As for the spinach, If you type in "Spinach hinder calcium birds" you will get a ton of websites telling about the problems birds can have with spinach.
Birds have a hard time absorbing calcium, hence why we have to give our ducks oyster shells. They can not absorb enough from just their food alone.
Because a deficiency in calcium could mean death for a female bird, (because the lack of calcium causes the lack of shells on eggs causing egg binding), spinach should not be given to female birds. I guess a bit every now and then or some for your males may not hurt your birds drastically, but do you really want to risk the lives of your ducks just because you want to feed them extra greens you may have?
I'm sure some people may have given their ducks spinach all the time and never had any problems, but i personally can not tell people it's ok to give their birds something that could harm them. As i said before, only give your ducks those items on that list at your own risk. Don't be surprised if your animals end up having a problem, because this food is KNOWN to cause problems.
Spinach also causes problem absorbing vitamin A as well as hindering calcium. That's two strikes against it. I just don't think it's wise to give your birds something that could harm them, but I can't stop anyone from doing so... Again, feed your animals what you think is best, but i'd rather give all duck owners the info about the bad effects of spinach.