I was going to say they sounded like Muscovies too! Congrats on the property and the ducks! 

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In the wild Muscovies roost in trees, safer from preds, and that maybe why you aren't seeing any ducklings, not only do they have to be concerned about land preds but in the air and under water pose a threat to them and their ducklings. Hawks, eagles owls and snapping turtles and large mouth bass will take a duckling and also injure adults. If you provide nesting boxes be sure to google info on what is the safest way of doing this, because a broody duck is a sitting duck. and a target for just about anything looking for a free meal.They bathe in the pond, forage in the grasses and hang out in the trees. They seem pretty happy, but I was wondering, is there anything I could add to make them more comfy? Do they like duck houses? And I haven't seen any nests or ducklings, so maybe I should provide nest boxes?
Did anyone tell you how long they have been there? yes snapping turtles are terrible on ducks and ducklings. If there is plenty of food and of course water they will probably stay. I don't know what your winters are like but they may need some help food wise during winter months to keep them healthy.I've looked in the trees where they hang out, non of which are really all that big (like 12-20ft, mostly), and haven't seen any nests. The trees don't seem to have any cavities in them, and I haven't noticed any nests in the crooks of branches either. That's why I'm considering the nest boxes. I research everything before I do anything, so if I decide to make some, I'll be sure they are up to par. The pond really isn't that big, maybe half an acre, and doesn't have any fish. I'm sure there are a few turtles tho. I didn't realize turtles would go after a bird. You learn something new every day. I just think these birds are really intresting and would love for them to stay. If they decide they can't raise ducklings here, they might leave, which would be sad.