Just out of curiosity....

[lshytle]

What happens if you do not add water at all except at the end of incubation to duck eggs. Trust me, I won't try this, I'm just curious.

 

[Farmer Viola]

Duck eggs are a bit different than chicken eggs, I believe they require more moisture (misting) throughout incubation but I have never done duck eggs personally. The 2 links in my signature have information about misting (first link) & the idea behind dry hatching (second link).

As for "what happens" if you run too low humidity, the eggs will evaporate faster. They may hatch faster because they lose water weight a little bit quicker. They may dry out and get glued to the inside of the shell on hatch day. Maybe someone else can add more about duck eggs.