It is often assumed that chick yield (the chick weight divided by egg weight) is influenced by moisture loss. Although at first sight that sounds not unlogical (more moisture loss could dehydrate the embryo and therefore result in a smaller chick so a lower chick yield) if we think a bit longer it is actually not that logical. First of all, the hen cant influence the relative humidity around the egg and therefore not the moisture loss. If moisture loss would influence chick yield, she would have probably found a way to regulate moisture loss to prevent dehydration.
If we look inside the egg, we see that the embryo is swimming in the amnion and dumps its metabolic water in the allantoic, which functions as a sort of waste bag. The allantoic surrounds the amnion after about 12 days of incubation and forms a layer between the amnion and the shell. This means that the moisture loss comes from the waste bag and not from the embryo itself, which of course is logical as it makes the embryo independent from a situation that cannot be influenced by mother hen, the relative humidity.
So why do people have the impression that moisture loss is influencing chick yield? When an embryo consumes the yolk, a gram of yolk results in approximately 0,65 g of embryo. This means that the more yolk is used, the lower the chick yield will be.
A low moisture loss (at a fixed relative humdity) is the result of low ventilation or low condutance. But not only moisture goes through the pores of the egg shell, but also oxygen. If the embryo gets less oxygen, it will not be able to convert a lot of yolk, as the use of the fat in the yolk requires a lot of oxygen. The result will be a higher amount of residual yolk and therefore a higher chick yield, above the optimum of 67-68%.
If we change the moisture loss by changing the relative humidity in the machine, we will see that the chick yield doesnt change. But if the temperature of eggs is changing because the cooling effect of the humidification system is not there anymore, we still might see a difference in chick yield, not because of the condutance of the egg but this time because we overheat the eggs and therefor have less yolk conversion.