Constant accuracy of humidity is less critical than that of temperature. Ideally, the egg needs to lose 13 – 15% of its weight between the time of laying and pipping. Fairly wide tolerances in humidity are bearable although not ideal, as long as the chick ends up having lost the correct amount of weight by the time of hatching. Correction can be made in later stages for errors earlier.

If the incubation humidity has been too high the egg will have lost too little moisture and the chick
will be rather large. In this case the air space will be too small, the chick’s respiration will be affected and the young bird will have difficulty breaking out of the shell because of the lack of space. Commonly with excessive incubation humidity the chicks will die having broken through the shell in one place (‘pipped’) either through weakness because of the lack of air to breathe in the shell or because of lack of space to turn and cut around the shell with their bill. Often, because of pressure within the egg, the bill protrudes too far out of the initial hole preventing the normal anti-clockwise progress of the bill chipping the shell from inside. The bill becomes gummed up with drying mucus.

Low incubation humidity levels lead to small chicks with large air spaces by the time the hatch is due. These chicks will tend to be weak and may also die just before, during or just after hatching.

It should be noted that in general that a slightly lower humidity level than optimum is likely to be less disastrous than a slightly higher than ideal level.

The weight loss rate will vary according the type of egg, the amount of ventilation, the handling of the egg, the breeder diet and the time in the season in which it was hatched. There are two guides that help indicate correct humidity.

Firstly, the air cell. The air cell increases as the incubation proceeds. In order to see this at first sight, the eggs need to be candled.

Candling lamps are lights with a concentrated beam that may be shone through the shell of the egg to illuminate the egg contents. This allows the size of the airspace to be determined which offers a guide to the weight loss rate. If the air space is larger than expected too much water is being lost and the humidity in the incubator should be increased to reduce the rate of water loss. If the air space is smaller than expected then the opposite applies.

Figure 1 (above): Diagram shows extent of airspace development throughout incubation (in days).

The other indicator is weight loss. If you weigh eggs before setting and weigh them as incubation progresses, the weight loss can be plotted on a graph (example below) to determine if the average weight loss has been correct. 

Humidity can be adjusted during the incubation period according to the graph reading at that particular time in the incubation process. As a general rule, if actual weights are lower than ideal, then humidity needs to be increased. If actual weights are higher than ideal, then humidity needs to be decreased.

There are two controllable factors that influence humidity levels. These are the amount of water surface area, and the amount of fresh air that the incubator draws in.

The greater the water surface area, and the less fresh air being drawn in, the higher the humidity levels inside the incubator will be. One method to increase water surface area is to use evaporating pads or sponges.

Finally, the environment in which the incubator is set up in can have an effect on accurate humidity control. If the ambient humidity in the air outside the incubator is very dry, then incubation humidity levels will be lower than if the air is very humid (wet). Also, cold air cannot retain much water vapor, so when cold winter air is warmed the

RH level will be very low. This happens in heated houses in winter, and also inside incubators. The result of this in general is that humidity levels tend to be lower in the winter than in summer and so humidity levels should be adjusted with this in mind.

The humidity levels required when a chick is hatching need to be higher than previously in the incubation period.

For the last day or so, high humidity levels are required to prevent the membranes of the egg drying too fast as the chick hatches and becoming tough and difficult to tear. The humidity level when hatching should therefore be at least 60%RH.

(Source: Brinsea)