Congratulations on the grouse but he does not look to be in good shape. That cage is not gamebird friendly and I don't think he'll last long in there. Are you working on more appropriate housing?
Yes, you'll need to remove all but one or two of the domestic ducklings- they grow so much quicker they will very soon out compete and inadvertently step all over the wood duck, likely killing sooner than later. Do it ASAP.
Also, please remember that this is a wild bird and what you're doing...
It's a Common/Black Scoter, a specialized sea duck species. It looks to be in pretty bad shape, but if it's not dead by now it should be brought to a professional.
Consider Ringed Teal. Unlike Wood Ducks and Mandarins they're in colorful plumage all year and less flighty. They're prolific breeders and the ducklings aren't terribly difficult to raise. Being non-native you don't need a federal permit either (you do for Wood Ducks). They also require little...
Do you have photos you could post? That would be helpful for identifying potential problems with the set up that could be hindering breeding.
Diet sounds fine, though you might want to offer lettuce and millet in the water as a treat. This will also encourage the birds to associate you with...
Without knowing anything about your birds, facility, diet, husbandry, etc., all I can say is that there are many variables that could prevent your birds from breeding. Nest boxes are most likely not the problem.
Domestic pigeons are like domestic ducks, chickens, etc. - a big mess compared to their wild counterparts. If you want to keep them I'd separate them from the aviary inhabitants. The vast majority of wild ducks don't trio, so get them in pairs.
Looks at others who have made hoop houses into...
Mandarins are very hardy birds. Most important to successfully wintering them is keeping open water available even in the coldest weather. There's lots of ways to do that- aerators, heaters, running water, etc. - choose the one that works for you.
If you have open water and some sort of wind...