I usually keep my humidity around 70% when hatching eggs. When a chick hatches, the humidity goes up by 10%!! Too high humidity can cause some chicks to drown and not hatch. Should I be worried about this every time a chick hatches??
Mine does the same as soon as a chick hatches but it settles back down within 20 minutes or so, so I leave it. If I get 4 or 5 popping out at once, or when there are alot already hatched then it goes up and stays up and then I open a vent for a bit.
Drowning with high humidity is an issue but I also find that after pipping the membranes get really stretchy if humidity is too high and the chicks can't turn to zip and eventually die, presumably from exhaustion
I try to keep it at 65% for chicks, 70% for ducklings, no more than 5% higher.