How Much Humidity Is Required During the Hatch?

One of the most common things people hear after a disappointing hatch is that the chicks "drowned" because the humidity was too high.
It's an easy conclusion to reach, but in reality it's one of the biggest myths in incubation.
What high humidity can do is reduce the amount of moisture an egg loses earlier, during incubation.
If the egg doesn't lose enough weight, the air cell remains too small.
When the chick finally breaks into the air cell to take its first breath, there may not be enough air available, and it can suffocate before it has the strength to hatch.
In other words, the problem isn't that the chick has "drowned".
The real issue is that the egg hasn't lost enough moisture throughout incubation, resulting in an air cell that is too small.
This is why I recommend monitoring egg weight rather than relying on humidity figures alone.
The egg itself tells you whether it is losing moisture at the correct rate, regardless of the humidity reading on the display.
When it comes to the hatch, high humidity is beneficial. See one of my articles below.

How Much Humidity Is Required During the Hatch?

The answer depends on the type of incubator.

In a multi-stage cabinet incubator, where eggs at different stages of development share the same cabinet, hatch humidity is always a compromise.
It must provide suitable conditions for the chicks that are hatching while not adversely affecting the younger eggs still incubating.
For that reason, the optimum hatch humidity varies with the species being incubated.
As a guide, I generally recommend around 60% RH for larger soft-feathered chickens, increasing to around 85% RH for species such as pheasants.

In a single-stage all-in/all-out incubator, where every egg is hatching together, the situation is different.
Here, I recommend raising the humidity as high as the incubator is capable of producing.
In general, the higher the humidity during hatch, the better, provided the chicks remain active and the cabinet is well ventilated.

Can Chicks Drown Because the Humidity Is Too High?

This is one of the most common misconceptions in incubation.
Immediately before hatching, the air cell inside the egg is already at 100% relative humidity. Although the humidity is 100%, it is still water vapour, not liquid water.
The embryo is also warmer than the surrounding incubator air, the air cell is slightly pressurised, and gases are continually moving outwards through the shell. Water vapour leaves the egg as part of this normal exchange.
For liquid water from outside the egg to enter the air cell would require it to move against the pressure gradient and through the shell pores in the opposite direction to the normal gas flow. Under normal incubation conditions, that simply does not occur.
High humidity therefore does not cause chicks to drown. Its purpose is to keep the membranes soft and pliable so that the chick can continue rotating inside the shell, complete the zip, and emerge without becoming trapped by dried membranes.

Another thing that adds to the confusion is what people see when they open an unhatched egg. What is often assumed to be water is usually something quite different.
It may be thick, sticky albumen (egg white) that hasn't been fully absorbed, which is commonly associated with incubation temperatures that have been too low.
Alternatively, if the embryo died some time before the egg was opened, the egg will have cooled and moisture in the air space can condense into liquid droplets.
In both cases, it's easy to assume the chick has "drowned", when in fact the liquid is either unabsorbed albumen or simple condensation, not water that has somehow entered the egg.