If the humidity is too high during the first 18 days condensation builds up in the air cell and when the chick pips the air cell it can drown. If the humidity is very low during the last 3 days the chicks can get stuck in their shell and unable to move to zip and hatch. If the humidity is...
That is a very good explanation. Where I live I incubate between 35% and 40% humidity. When I go into lockdown I increase it to around 70% to 75%. I played with the humidity and this give me the best hatch rates. I have had 100% hatches.