Humidity in the incubator is the most confusing part of hatching chicks. Unlike temperature, where you can aim for a number that you know works, RH is, well, relative. Relative humidity (RH) is a percentage of moisture present on the air compared to it being fully saturated. Technically, you can’t go more than 100%, though I have seen RH meters show higher than 100.
Why is RH important? During incubation, eggs with live chicks release water vapor and CO2 to the atmosphere. This is normal and necessary or the chicks will drown. If the air is too dry (low RH) then they lose water too rapidly, dehydrating sometimes to the point of death. If the air is too wet (high RH) then they risk drowning, or not being able to maneuver into a position to hatch.
So what is the right RH? It really and truly depends on a number of variables:
- Porosity of the egg shell – ever noticed some eggs have a glossy surface, while others are more matte? They lose water at different rates at the same RH. Eggs laid early in the hen’s cycle tend to be less porous than later. Some breed have different shell types. I believe that the dark eggs, especially those with a glossy finish, like some Marans and Welsummers, lose water less readily and so need a lower RH.
- Size of the eggs – This is a simple surface area to volume ratio thing.
- Age – Eggs that have been stored for a while have already lost some moisture to the air, before they even get into the incubator.
- Washed vs unwashed – Washed eggs lose some or all of their “bloom” that protects them from moisture loss.
There may be other factors, but you can see from this list that arriving at the “right” RH for a mixed group of eggs is pretty much impossible. So what to do?
Experience is the best teacher here, and I mean experience with a specific incubator and specific breeds of poultry. Some breeds/hybrids are so vigorous and easy to hatch that any RH seems ok, others are more finicky. Peafowl tend to need a much higher RH during the setting phase (before “lockdown” or moving to the hatcher. Coturnix quail want a lower RH during all stages. Turkeys need lot RH during setting and very high for hatching. You get the idea.
Generally, you can only add water to increase RH, if you remove the water and the RH is still too high, that is hard to manage. So, I start out with low RH and increase it if needed (if I get a bad hatch). During the course of the hatching season (Winter to Summer mostly), the RH trends upward no matter what I do. Nice that that correlates to the more porous shells of summertime.