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

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

Zone of suspended

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!