and for what its worth, my flock is in my sig. I usually have between 8 and 12 ducks, typically two drakes. I also, for reasons not important here, feed my birds in ways I don't recommend the typical backyard owner do. That's a mix that ends up at 20% protein, but around 2.8% calcium.
My sample size is so small as to not be worthy of being called anecdote, much less data, but I have had no deaths attributed to calcium toxicity. However, when I've culled older drakes (12-15 mo) to make room for fresh breeders. there have been visible signs of the progression on the intestines and liver. I've not closely inspected the kidneys, but assume the signs were present there as well. one of these days, i'll take a photo