During incubation I use low humidity with chicks, I dry hatch, in my house I can run 20 to 40% easily with a full load of eggs in the incubator without adding water, some recommend 50 or 55% but I have noticed even with my lower humidities sometimes the air cell isn't as big as I expect, with the 50 and higher I think I would have drowned chicks for sure. With ducks it may differ I'm not sure. During lockdown I filled my reservoirs with water and got around 70 then as chicks hatched and exposed their wet fuzz to the incubator's air the humidity skyrocketed to the point that the windows were fogged with moisture and I could hardly see inside, high humidity during the hatch is fine as by that stage the air cell is done shrinking and the humidity's only purpose is to keep the shell membrane damp enough to not shrinkwrap the chicks or dry and toughen up to the point that the chick cannot zip it open. My next hatch I am only going to fill half of the reservoirs and try to prevent the fogging of the windows, the humidity doesn't need to be 80 or 90% even for the hatch but I don't believe it hurts anything either. Some even recommend dropping the temp a degree or so during hatch but I don't because the temp control is so touchy on these small incubators I don't want to lower more than I mean to.
