There is some discussion and disagreement about drakes and calcium. Some are more concerned than others about that.
Dandelions are said to have quite a bit of calcium. So are collards and kelp.
I dissolve calcium citrate tablets in water and put that into the layer feed. Here is a recent wrinkle, though - for nearly two years I have kept three of the Buffs (a drake and two females) separate from the rest of the flock of Runners and one Buff. This winter, peace has been made between the two groups.
So, now, I am sorting out how to give him less calcium than I give the rest. His females did not have calcium issues, so I fed them all layer with no extra calcium.
I have only decided so far that the risk of egg yolk peritonitis and prolapse are greater than the risk of problems for the drake. If someone feels vigorously otherwise, I respect that. I have talked this over with my vet. And if there is research on ducks on this topic I would be happy to learn of it. The peer-reviewed article I am aware of is for a different species of bird.
One scientific article I read indicated that ducks have the ability to store and release calcium that is diminished by giving them a consistent amount of calcium daily. In case that applies to my flock, I have been varying the amount of calcium I give them.