N740 US Issue 01
3
Technical Notes
Humidity in Incubation
No aspect of incubation causes more confusion and concern than humidity. When other factors appear correct
humidity is often blamed for poor hatches but whether too high or too low may still be in doubt. It doesn’t
have to be so; a few simple procedures can take the mystery out of humidity and put control back in the hands
of the operator.
Humidity is one of four primary variables which must be controlled during egg incubation - the others being
temperature, ventilation and movement (turning). Humidity is the most difficult of the four to monitor
accurately and to control and therefore is commonly misunderstood. The operator instructions that accompany
all incubators give guidelines to achieve correct humidity levels for most species under normal conditions and
in the majority of cases this gives excellent results so first check that you have followed these guide lines.
However there are times when incorrect humidity levels do cause problems and further steps are needed to
check that humidity levels are correct. This article explains the effect of different humidity levels, measurement
of humidity and the best techniques for achieving correct humidity levels.
Before spending time and effort checking incubation humidity levels it is essential to ensure that temperature
and egg turning are correct - refer to the unit’s operating instructions. Also check that the eggs are fertile and
the parent stock healthy, properly fed and free from in-breeding.
The effect of humidity upon the incubating egg
Egg shells are porous - they allow water to pass through, and so all eggs, whether being incubated or not, dry
out slowly. The amount of water that an egg loses during incubation is important and this is determined by the
humidity levels within an incubator; if the humidity level is higher then the egg will ‘dry out’ more slowly than if
the humidity is lower.
All eggs have an air space at the round end and as water is lost through the shell it is replaced by air drawn
through the shell into the air space which gradually increases in size. This air space plays a crucial part in
hatching. It is the first air that the fully developed chick breathes and the space allows the developed chick
some movement inside the shell to allow it to manoeuvre into hatching position.
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 excess incubation
humidity chicks will die just before or after 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 in
general that a slightly lower humidity level than optimum is likely to be less disastrous than a slightly higher
than ideal level.
It is important also to understand that humidity does not directly affect embryo development unless the egg is
seriously dehydrated. Only temperature and egg turning affect growth of the embryo directly. Humidity is
important only to achieve the right balance between excessive dehydration and space within the egg as it
reaches full term. Thus a temporary error in humidity can be corrected later provided the error is observed
and the right action taken. Death of an embryo at early or mid term stages of incubation is not usually
attributable to incorrect humidity.
