It is not the best thing to feed layer pellets to non-laying birds, and at the same time, many people do with no problems.
There is a concern that too much calcium is rough on the kidneys. Yet I have also read that ducks can store and release calcium, self-regulating. So maybe some ducks can do that better than others. It would be nice to have some more research into this.
Meanwhile, another risk is that if the drakes start feeling like mating, if they try to mate with a chicken it could kill her because their bodies are so different. Please please be aware of that and since spring is coming, you may see overnight changes in the drakes' behavior - know that and step in when necessary.
