This is a question frequently asked by anxious incubationists, usually after the event which
came unexpectedly and the query is therefore ‘what damage is likely to have been done?’
Occasionally the power shutdown can be predicted and the concern is to keep damage to a
minimum.

With the emergency situation in mind as well as the more subtle question about daily cooling
of eggs during incubation, we have attempted to set out some of the more fundamental
research information together with our own experiences and suggestions.

A review by H. Lundy of research carried out by a number of scientists over many years
identified five temperature zones each of which is characterized by its major affect on the
developing embryo. These zones are not clear cut. There is some overlapping and the time for
which the embryo is exposed and the age of the embryo blur the limits.
Lundy’s five incubation Temperature Zones:-

Zone of heat injury Zone of hatching potential Zone of disproportionate development Zone of suspended development Zone of cold injury

In common with most scientific work on incubation, this data assumes an incubator with a fan
(virtually no temperature differences within the incubator) and was based on chicken eggs.
These zones are further explained as follows:

Zone of heat injury (above 40.5°C/104.9°F)

At continuous temperatures above 40.5°C (104.9°F) no embryos would be expected to hatch.
However the effect of short periods of high temperature are not necessarily lethal. Embryos
up to 6 days are particularly susceptible, older embryos are more tolerant. For example,
embryos up to 5 days may well be killed by a few hours exposure to 41°C (105.8°F) but
approaching hatching time they may survive temperatures as high as 43.5°C (110°F) for
several hours.

Zone of hatching potential (35 - 40.5°C/104.9 - 84.5°F)

Within a range of 35 to 40.5°C (84.5 - 104.9°F) there is the possibility of eggs hatching. The
optimum (for hens) is 37.8 °C (100.4°F), above this temperature as well as a reduced hatch
there will be an increase in the number of crippled and deformed chicks. Above 40.5 °C
(104.9°F) no embryos will survive.

Continuous temperatures within this range but below optimum retard development and
increase mortalities. However it is again evident that early embryos are more susceptible to
continuous slightly low temperatures than older embryos. Indeed, from 16 days on it may be
beneficial to lower the incubation temperature by up to 2°C (3.6°F). I emphasis the word
‘continuous’ because the effects of short term reduction in temperature are different and are
discussed later.

Zone of disproportionate development (27 - 35°C/80.6 - 95°F)

Eggs kept above 27°C (80.6°F) will start to develop. However the development will be
disproportionate in the sense that some parts of the embryo will develop faster than others
and some organs may not develop at all. Below 35°C (95°F) no embryo is likely to survive to
hatch. Typically the heart is much enlarged and the head development more advanced than
the trunk and limbs.

The temperature at the lower end of this range is sometimes referred to as ‘Physiological
zero’ - the threshold temperature for embryonic development. Unfortunately different organs
appear to have different thresholds resulting in an unviable entity.
Zone of suspended development (-2°C - 27°C/28.4 - 80.6°F)
Below about 27°C (80°F) no embryonic development takes place. Prior to incubation, eggs
must be stored in this temperature range (preferably around 15°C/59°F).

Zone of cold injury ( -2°C/28.4°F)

Below this threshold ice crystals will start to form in the egg and permanently damage may be
done to internal structures. Eggs may lie for some considerable time in temperatures close to
freezing without suffering damage.

The analysis above gives us a fair idea of what may be happening to embryos kept
continuously or for long periods within these temperature bands. Of course continuous
incubation at any temperature other than near optimum is of little practical interest because it
will not result in live birds but this information does give a better understanding of what may
happen if eggs should be accidentally overheated or chilled.

Further scientific data has resulted from experiments concerned specifically with intermittent
chilling of eggs. There is evidence that, during the early phase of incubation, chilling of eggs
to below ‘physiological zero’ (say 25°C/77°F) does less harm than chilling to temperatures
above that level. Embryos up to 7 days old may well survive cooling to near freezing for 24
hours or more without damage.

The cooling delays hatching but not by as much as the period of chilling - so there appears to
be some degree of compensation. The older the embryo, the more likely it is to die as a result
of chilling to below 27°C/80.6°F but the effect on surviving embryos is not detrimental.
Other experiments have concentrated on cooling eggs less severely to temperatures within the
zone of ‘disproportionate development’. In virtually all such experiments, increases in
hatchability have been reported varying from 2% to 25%, or even higher in the case of ducks
and geese. There is some doubt as to whether the effect is due to changes in humidity, CO2
level or to chilling alone.

A number of conclusions from this data which have practical implications:

1. Cooling eggs for short periods, say 30 to 40 minutes, on a regular basis (say once
every 24 hours) at any stage during incubation has no detrimental effect and is
probably of benefit.

2. If eggs are likely to be cooled for longer periods (more than 2 hours) the way they
should be treated depends upon their state of development. If the eggs are newly set
the best plan is to cool them fairly quickly down to 5 - 20°C (41 - 68°F) and hold
them in this range - put them in the fridge!

It may also be best to treat eggs this way up to about the 14th day, although greater
losses must be expected if severe cooling occurs later in incubation.
If power loss occurs when the eggs are near hatching, incubator temperature is less
critical, but severe chilling will cause mortalities. It is preferable therefore, to take
reasonable steps to limit heat loss by keeping the incubator shut and raising the
temperature of the room if possible. The metabolic heat from the embryos will keep
them warm for quite a long time.

3. Avoid maintaining eggs in early stages of incubation for long periods of time in the
‘zone of disproportionate development’ (27 - 35°C/80.6 - 95°F). This will result in a
large number of deaths and abnormalities.

4. Avoid subjecting the eggs to over-temperature at any time but particularly in the early
days of incubation.

Remember that incubator thermometer readings will not be the same as embryo temperatures
when cooling or heating occurs. The eggs will lag behind the air temperature. For example,
cooling hens eggs by taking them out of the incubator into a room at 20°C/68°F for 30-40
minutes is likely to cool the internal egg temperature by only 3 - 5°C (7 - 10°F). Eggs smaller
or larger than hens eggs will react quicker or slower accordingly.

There is very little data on the effects of cooling eggs of other species. Duck eggs and to an
even greater extent, goose eggs, are said to benefit from periodic cooling. Our own experience
seems to confirm this and we know of instances where the eggs of both duck and domestic
geese have been subjected to severe cooling for prolonged periods without harm.

There is an obvious analogy with the natural process in cooling eggs periodically. Most
species of bird leave the nest for short periods to feed. It is quite possible that the resulting
cooling and re-heating provides a stimulus to the embryo, which actually encourages growth.
If the effect is more pronounced in ducks and geese it may be because the requirement has, to
some extent, been bred out of hens by years of artificial incubation. It would follow that
totally wild species may be even more susceptible to a cooling stimulus. Certainly there is no
evidence to suggest that short term cooling is likely to be harmful.

Hopefully these explanations will enable bird breeders to assess the likelihood of damage
from accidents. It should certainly allay any fears about the cooling that may accompany the
manual turning or inspection of eggs!

Humidity is one of four primary variables which must be controlled during egg incubation - the others
being temperature, ventilation and movement (or turning). Humidity is the most difficult of the four to
measure accurately and 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 most cases this gives excellent results so please 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 information sheet 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 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 – the greater the water loss
through the shell, the larger the airspace. This air space plays a crucial part in incubation. Within 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 maneuver 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 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.

Measurement of humidity.

Many materials are capable of absorbing water or water vapour and air is one of them. Water vapour is
a gas like any other gas, and air is a mixture of gases, one of which is usually water vapour. The
difference is that the amount of water vapour varies widely whereas the other gases which make up
our atmosphere remain fairly constant. The range of vapour may be from none to a certain maximum
which the air can absorb (called saturation). This maximum increases with temperature.

There are two commonly used ways to measure humidity and the differences need to be clearly
understood. These are:

Relative Humidity (RH) expressed as a percentage.

This is a measure of the amount of vapour in air compared with the maximum that could be absorbed
at that particular temperature. This is why relative humidity (RH) is quoted as a percentage. For
example an incubation RH level of 50% might be quoted. This means that at incubation temperature
the air in the incubator contains half of its maximum possible water vapour capacity. Because
maximum possible water content increases at higher temperature, if the temperature was increased but
no additional water added then the % RH level would drop.

A good way of imagining this effect is to think of a bath sponge. When the sponge is squeezed to half
it’s normal size clearly it can hold less water. Imagine a half squeezed sponge soaked in water until no
more can be absorbed (saturated) this is analogous to cold air at 100% RH - no more water can be
absorbed. If the sponge is allowed to expand completely then, although the amount of water has not
changed, the sponge is relatively dryer than before because it has greater capacity to absorb water.
This is analogous to warmer air containing the same amount of water vapour which will now have a
much lower RH level. Conversely when air cools the capacity of the air to hold water vapour reduces
and % RH levels will rise. If the air temperature drops below the saturation point (100%RH) the water
vapour condenses. An example of this is dew forming on a cold night after a warmer day.

Wet Bulb temperature

This is the temperature (in degrees C or F) of a thermometer with a moist cotton wick around its bulb.
Evaporation of water from the wick cools the bulb by an amount related to the relative humidity. This
cooling effect is the same as the chill we feel when we step out of a shower. It is the difference
between Wet Bulb temperature and air temperature that is important, so air or Dry Bulb temperature
must also be known to define the RH. In incubators the Dry Bulb temperature is constant (we hope!)
so WB is often quoted on its own.

Direct measurement of RH is not easy. Cheap hygrometers are available but you get what you pay
for; we have seen cheap instruments reading 30% different from out of the same new pack! More
expensive direct reading digital instruments are better but need re-calibrating regularly. When looking
into digital hygrometers check both the accuracy quoted and the hysteresis percentage. Both figures
should be better than +/- 5% - if either is not quoted, don’t buy it! For example: Brinsea’s H22
Humidity Management Module uses a top quality sensor with accuracy of +/-3% and 0% hysteresis –
see below for more details.

The most reliable, cheap method of measuring RH, is to measure wet and dry bulb temperatures and
convert the information to %RH by using a simple chart.

A couple of points worth noting. The Wet and Dry bulb thermometers may be conventional (mercury)
thermometers or they may be electronic sensors. There are two special cases where Wet and Dry bulb
readings are identical; when the air is saturated (100%RH), and when the wet wick has dried out!
A further complication is that it is difficult to measure humidity in ‘still air’ incubators. Wet bulb
thermometers do not work well in near static air conditions. The other problem is that the temperature
will vary by several degrees from the top of a still air incubator to the bottom and so RH readings will
vary with height too. Fortunately the humidity level in still air incubators is probably less critical than
fan assisted (or forced draught) machines (see information sheet ‘So why fit a fan?’).

Brinsea Products offer web bulb thermometers to suit our range of moving air incubators. Contact you
local stockist or Brinsea Products direct for more details.

Achieving correct humidity levels

There is a fairly easy and reliable way of measuring RH indirectly and directly measuring the effect
that RH level has on the egg. This is by weighing the eggs to monitor their water loss over the
incubation period. Most species of bird (with the exception of the ostrich family) need to lose between
13 and 15% of their weight from the time of setting the eggs in an incubator to hatching. By
measuring the weights of the eggs at intervals during incubation, taking the average weights and
comparing these to the expected weights needed to achieve the ideal weight loss by hatching, it is
possible to see when the rate of water loss is too great due to humidity being too low and vice versa.
In practice this means drawing a graph (see below) with incubation time in days along the x-axis and
weight up the y-axis. The average weight of eggs when set (day 0) can be entered and the ideal
hatching weight (average day 0 weight less 14%) can be plotted on the day the hatch is due. These two
points are then joined to give the ideal weight loss line. Average weights can then be taken every three
or four days and plotted on the graph. If the actual average weights are lower than the ideal then
humidity levels need to be increased and vice versa. Thus any deviation from the ideal weight loss line
can be corrected as incubation progresses. The important point is to reach the ideal weight loss by
hatching day; some deviation form the ideal weight loss line earlier in incubation will have little
adverse effect.

The graph above shows the average actual weights of incubating eggs against the ideal weight loss line - Note that the greater than ideal weight loss in the earlier stages of incubation has been corrected by hatching day.

The combination of monitoring egg weight loss and precise control of humidity with the Automatic
Humidity Management Module (see below) is the ultimate solution of ensuring correct incubation
humidity.

Altering incubation humidity levels

All incubators should have the facility to evaporate water inside the egg chamber and thereby
influence humidity levels. Always refer to the manufacturers instructions. The important point is that
two controllable factors influence humidity levels: water surface area and the amount of fresh air the
incubator draws in. All Brinsea incubators have two water vessels to give some flexibility over
evaporation rates. Remember that it is the total surface area of water that matters not the depth. So to
increase humidity levels fill the second vessel (or if both are dry, fill one) and reduce ventilation by
either adjusting the control or blocking up to half of the ventilation holes. Some ventilation must be
maintained to allow the chicks to breath. Refer to the operator instructions for your model. In
exceptional circumstances it may be necessary to further increase the surface area of evaporation by
using evaporating pads or blotting paper to soak water from the vessels in the incubator. Do not spray
the eggs with water - the increase in humidity is very short lived and bacteria may be spread.
A third factor does affect incubation humidity levels and this is the ambient (or environmental)
humidity level. Clearly if the air being drawn into the incubator contains very little water then
incubation humidity levels will be lower (all else being equal) than if outside air is very humid. As
explained above cold air cannot contain much water vapour so when cold winter air is warmed
temperature the Rh level will be very low (remember the sponge!). This happens in heated houses in
winter and in incubators. The result is that, in general, humidity levels will tend to be lower in your
incubator in winter than in summer and so water evaporation and ventilation levels should be adjusted
with this in mind. Because eggs are particularly sensitive to excess incubation humidity the most
common mistake associated with incubation is to use the same regime of water and ventilation in the
summer that was successful in the winter. In warm summers it may be possible to add no additional
water to the incubator until hatching time because the combination of warm, damp ambient air plus the
water given off by the eggs themselves gives sufficient RH levels.

There is no evidence of any change in ambient humidity levels associated with global temperature
change as a result of the Greenhouse Effect. Small climatic temperature changes are insignificant
when compared to seasonal variations and so although it may be fashionable, there is no justification
in blaming a poor hatch on global warming.

Humidity and Hatching

The humidity levels required as the chick emerges are different from those earlier in incubation. For
the last day or so of incubation humidity levels need to be much higher than earlier on. By this stage
the weight loss of the egg should be 13-15% and water loss for the last 24-48 hours will not
significantly affect this. The high humidity levels are required to prevent the membranes of egg
drying too fast as the chick hatches and becoming tough and difficult to tear. In natural incubation the
membranes cannot dry quickly because the parent bird is sitting on the egg but in an incubator drying
membranes can be a problem. The actual level of humidity is not too critical for hatching but needs to
be at least 60% RH. Humidity levels drop rapidly when the incubator is opened and take much longer
than temperature levels to re-establish. Try to avoid the temptation of opening the incubator too often
when chicks are emerging to maintain high RH levels.

Automatic Humidity Management

To meet the needs of bird breeders concerned about controlling incubation humidity Brinsea have
introduced the Automatic Humidity Management Module. This device serves to provide a highly
accurate and constant readout of humidity (expressed in %RH) and an a precisely controlled pump
which regulates RH within the incubator to the setting the user selects. Thus, in a similar way to the
principle of Brinsea’s temperature controls, the user turns a knob to select the humidity level the eggs
require, the unit responds by altering the amount of water pumped to the incubator and the change in
humidity level can be monitored on the meter. Because the system is constantly monitoring the
incubation humidity level, external effects, such as seasonal ambient humidity variations, are
compensated for and the incubation humidity level remains constant. For hatching, the user simply has
to increase the setting on the module and the new setting will automatically be maintained. The
Automatic Humidity Management System overcomes problems of wicks drying and becoming
contaminated sometimes associated with wet bulb thermometers and provides the ultimate in
refinement of humidity control. Versions are available for all forced draught (fan assisted) models in
Brinsea Products’ range of incubators. Contact your stockist or Brinsea Products direct for more
details.