generally there are two arguments that keep cropping up, the one side says keep things as dry as possible to avoid chicks drowning, the other side says humidity can be a wide range as long as the eggs don't dry too much or chicks will get stuck to the shell or get shrink wrapped.
So those are the two arguments and I feel every person incubating for the first time needs to pick a side they want to support. That is what I have found it pretty much boils down to.
Now you can go into more technical aspects and discover that some eggs have thicker shells and lose less moisture so a lower humidity is beneficial for them, other eggs might do better with a higher humidity, eggs lose moisture and increase the humidity in an incubator themselves, so if an incubator is filled all the way with the eggs you might need less water in the water tray and if the incubator has few eggs you might need to add more water to compensate.
So that is why every advice can be right for certain situations but it might or might not be right for yours.
The best way is to candle the eggs and monitor the change in size in the air cell every week. As the embryo develops it will absorb and transpire fluid and the air cell will get bigger as a result, the size of this air cell will show how the egg is developing, whether it is losing the right amount of moisture, too much or too little and then humidity can be adjusted to adjust this rate of evaporation if needed.
I will add that having been on BYC for a few months I can fairly safely say I see far more cases of chicks getting stuck hatching than chicks drowning.
So I personally see a too low humidity as far more dangerous than a too high humidity which I've seen cause no problems for people.
I think it is too easy for people to have a bad hatch - which happens to all of us - and want to find something to change and humidity is the easiest and simplest thing to change. I honestly don't quite understand why there is such a big divide when it comes to what humidity to use.
Maybe it does just come down to the egg shell thickness.
But as a last note I will add that when you are asking the community what humidity they would advise, know that you are asking everyone from commercial breeders, to small incubator hobbyists, from dry air incubators to forced air incubators.
There will be huge differences between these, like I have concluded by now that a forced air incubator has the fan circulating the air like a hair dryer. What does a hair dryer do? It dries hair so logically a forced air incubator will dry eggs more than a still air incubator and maybe it is just as simple as a still air incubator needs a lower humidity for the eggs to lose the right moisture and in a forced air the fan is doing that already so humidity should remain higher.
There are a lot of factors at play and you have to decide which is most likely best for you.
I hope that has cleared up a bit of the confusion for you, for lockdown eggs need roughly 65-70% humidity, a day here or there won't change things too much - as long as the eggs don't dry excessively so you shouldn't have anything to worry about as you will be increasing humidity soon.
