Good thread but a lot of misinformation on here. [Any quotes below will be from Dr. A.F. Anderson's renown masterpiece & biblical authority on this subject, The New Incubation Book , 1979 & revised thereafter(252 pp)]
"The optimum [humidity] for incubation is about 50-60%." A lot of us reside in locales with high humidity anyway & depending on where we have our incubators makes a big difference too.
First, about mother hens & nature: Humidity has been recorded under different hens & it "showed enormous variations, depending on the site of the nest, the prevailing weather conditions, and whether the bird frequented water or not. The average figure (under a hen during natural incubation) is about 60% relative humidity." The best place for a natural nest is on the ground and the humidity doesn't come from the hen but from the ground. Obviously, this can vary greatly where you live. Here in Alabama, we have very high humidity and during the summer, my hens have hatched 100% even with 70%+ average humidity outside (and it raining all the time). Once, my hen stole her nest by the side of the house on the ground refusing to move anywhere else so I let her sit there through rain or shine-- I put a cardboard box over her when it poured. She hatched all.
Sometimes, if a hen is setting in the shed or up off the ground level and not getting the ground moisture, it may be necessary to add some moisture to the nest-- I always look at the size of the air sac -- if too small, then I don't; if too large, then I mist them. This is the best way monitor your humidity-- candling the egg.
"There is no doubt that the broody hen will beat most incubators, and it is in the first few days that she does it." That first week is the most critical time.
Also, broody hens vary-- A hen with tight feathering and more meat on the breasts make "hot hens" and a hen carrying the meat on their legs and fluffy legs and looser feathers incubate "cooler." "Generally, the bigger the egg, the hotter the hen needed."
Second, chicks don't drown in their eggs. In fact, they can be rather forgiving of you getting the humidity wrong: "It is a paradox that there is an optimum relative humidity level in successful incubation, but that this can fluctuate between very wide limits, to a far greater degree than the egg could tolerate in temperature variation. This is because the embryo does have some control over its water metabolism.
During the last third of its time in the egg, the embryo swallows the amniotic fluid and remains of the albumen that now mix with it. It can thus thus drink its way out of trouble, or not drink if it be too wet."
Many peoples' flocks become "adapted" to that person's incubator or technique & their success rate will climb over time (sort of like Evolution's natural selection). The chicks that hatch from the incubator were successful or they wouldn't have hatched. Their eggs and their chicks eggs also hatch from the incubator -- i.e. where a lot of the commercial industry eggs (whose strains have adapted over decades to the large commercial incubators) would not hatch as well under my hen naturally (or their success rate would certainly not match the commercial incubator)-- IT IS THE SAME WITH US!!--
Another general argument I'd make is that some folks have successful hatches in spite of doing things wrong & so they think that their methods are correct. Many think that if an egg is fully developed at the end and dies having pipped or not pipped, then it "must" be something they've done"wrong" at the end time-- the last few days (they'll say the chick drowned because the humidity was too high at the end-- when actually, folks are helping chicks out because it is too dry and the shells stick to the chick). However, what they don't realize, is that the chick was alive but was doomed from early on & never meant to hatch. If they got their eggs from someone else, they don't know the management practices of the breeder, handling of the eggs, storage (and that might have doomed the hatch's success (%) rate):
"Death before the establishment of pulmonary respiration: The change to breathing with its lungs is one of the biggest steps that the embryo makes. Any weakness, either inherent in the egg or caused by adverse conditions imposed upon it, will cause a high mortality at this stage. All too often, at post mortem the chick appears to be perfectly developed; it just died instead of starting to breathe. The beak has not entered the air cell.
All the faults of parent stock management, heredity, egg storage, hygiene, and disease make their mark now.
. . . It is important to remember that mistakes made earlier in incubation, such as temporary over -heating, chilling, and failure to turn in the early days, although not appearing to kill the egg at that stage, have so weakened it that it dies now."
"Death after breathing has been established: These are usually referred to as dead in the shell. The chick may or may not have chipped the shell. All the causes of a weakened embryo, either inherent in the egg or produced by poor incubation techniques, will cause death at this stage as well as before breathing commenced."
Temperature (too high and/or fluctuating) is one of the main culprits of bad hatches and not so much humidity.
Personally, I have tried the Dry Incubation Method VS. simply following the directions that came with the incubator & filling the trays accordingly AND I found my hatch rate HIGHER with the later (in both still air and circulated air models). My 100% hatches have been in my still air with me hand turning and with my OEG hen.
Long & short of it: If all other environmental conditions are highly favorable (i.e. steady & good temp; good parent stock with a good diet, etc.), then all your chicks may be so strong that humidity, wrong or not, they're going to make it.
