Most incubation problems are decided before the setter door closes. By the time an operator is chasing a temperature curve on day 12, the batch has usually already been shaped by how the eggs were collected, stored and warmed.
Key takeaways
- An eggshell temperature of 37.8 °C (100 °F) is the accepted optimum from set to transfer. At that value the chick hatches around 504 hours, roughly 21 days.
- Eggs should lose about 11 to 13 % of their weight by transfer at day 18. Relative humidity is only the tool used to reach that figure.
- Beyond seven days of storage, hatchability falls by roughly 0.5 % per day, against about 0.2 % per day before that point.
- Commercial setters turn eggs hourly through about 45°, and turning stops at transfer. Days 3 to 7 are the most sensitive to a turning failure.
Before the setter: the eggs already carry most of the result
Storage is the stage most often treated as logistics rather than as incubation, and it is where a hatchery quietly loses points it cannot recover. Published work on hatching egg management is consistent on the shape of the curve: every day of storage up to seven days costs around 0.2 % of hatchability, and beyond seven days the loss accelerates to roughly 0.5 % per day, with figures up to 1.5 % per day reported for long storage under less favourable conditions.
Where long storage cannot be avoided, the technique documented by Lohmann Breeders under the name SPIDES, short periods of incubation during egg storage, is worth knowing. Eggs receive three to six hours at incubation temperature during the first days of storage, most hatcheries working with three to four hours. The stated rationale is that the germinal disc advances to a stage of roughly 60 000 to 80 000 cells, where it tolerates storage better. This is a protocol to set up with your breeder company and your own data, not a setting to improvise.
Eggshell temperature, not machine temperature
The single most useful conceptual shift for anyone moving from a tabletop incubator to a professional setter is this one: the number on the controller describes the air, and the embryo does not live in the air. Eggshell temperature is used in commercial incubation as the indicator of embryo temperature, and 37.8 °C (100 °F) is widely accepted as the optimum from the start of incubation to transfer, according to technical documentation published by Petersime.
The consequence is directional and easy to observe. Below that value, development slows and the hatch runs long. Above it, development accelerates and chick quality suffers even when the hatch percentage looks acceptable. Embryos also generate their own heat from roughly the second week, which is why a setting that was correct on day 3 can be too warm on day 15 in a machine without adequate cooling capacity. Infrared measurement on the shell, or automatic adaptation of air temperature to measured shell temperature, exists precisely because the air reading alone is not the answer.
Is 37.5 °C or 37.8 °C the right figure?
Both numbers circulate because they describe different things. Values around 37.5 °C are commonly given as an air set point for small forced-air incubators, while 37.8 °C is quoted as an eggshell temperature target in commercial practice. Comparing them directly is the mistake. Establish which of the two your equipment actually measures before adjusting anything, and follow the incubation guide published for your genetics.
Humidity is a means; water loss is the target
Setting a relative humidity value and hoping is the most common cause of drowned or dehydrated embryos. What matters is the cumulative loss of water through the shell, and the industry benchmark is stable across sources: eggs should lose about 11 to 13 % of their initial weight from set to transfer, with Cobb technical documentation citing a comparable band of 10.5 to 12.5 % between lay and transfer at 18 days.
Weighing a marked sample tray at set, day 10 and day 18 turns humidity from a guess into an adjustable variable. If the sample is losing too much, raise humidity; too little, lower it. The measurement plan below is what we would ask any hatchery to be able to produce.
| What you measure | Reference value | What a deviation is telling you |
|---|---|---|
| Eggshell temperature | 37.8 °C from set to transfer | Long hatch if low, early and poor quality hatch if high. |
| Cumulative weight loss at day 18 | 11 to 13 % of initial weight | Below range, air cell too small and chicks drown at pipping. Above range, dehydration and sticky chicks. |
| Storage duration before set | Ideally under 7 days | Each extra day beyond 7 costs roughly 0.5 hatchability points. |
| Turning up to day 18 | Hourly, around 45° | Early dead embryos adhering to the shell membrane, concentrated in the first week. |
| Spread of hatch | Most chicks within 24 hours of the first | A hatch dragging past 36 hours points to uneven temperature or mixed egg ages. |
Turning: frequency, angle, and the week that decides it
Turning prevents the embryo from adhering to the shell membrane and supports the development of the extra-embryonic membranes. Commercial setters turn eggs hourly through roughly 45°, which amounts to 24 turns a day, and turning continues until eggs leave the setter at day 18. Technical reviews single out days 3 to 7 as the period where a turning failure does the most damage to hatchability.
Is manual turning three times a day enough?
It produces hatches, and it is what most small-scale operations do, but it is not equivalent. Marking each egg with an X on one side and an O on the other remains the simplest way to be certain every egg was actually moved, and an odd number of daily turns avoids leaving eggs in the same position every night. If you are running a manual machine, treat frequency as the variable to increase first when hatchability is unsatisfactory, before touching temperature. Our guide to artificial egg incubation step by step covers the manual routine in more detail.
Transfer, lockdown and the hatch window
At day 18 the eggs move from the setter to the hatcher, turning stops, and the chick begins orienting itself towards the air cell. From that point the batch should be left alone: opening the machine during pipping causes a humidity drop at the exact moment the membrane must stay pliable.
The hatch window is the quality indicator most often ignored. Chicks that hatch first wait without feed while the last ones are still pipping, and a window that stretches beyond 36 hours means part of the batch will leave the hatchery already behind. A wide window usually reflects temperature variation inside the machine or a set that mixed egg ages and flock ages, not a fault in the hatcher itself.
What the broody hen still teaches
The natural model is not nostalgia; it is a specification sheet. A broody hen holds a stable temperature by adjusting her position, keeps a humid microclimate under her body, and turns the clutch continuously. She also does something no machine does, which is to vocalise during the final days while the chicks call back from inside the shell, contributing to the synchronisation of the hatch.
That last point is the one worth borrowing. A tight, synchronised hatch is a natural outcome when every egg experiences the same conditions and the same age. Reproducing it industrially is mostly a matter of uniformity: uniform egg age, uniform egg weight, uniform temperature across every position in the machine.
Where hatchability is actually lost
- Fertility, not incubation. Candling at day 7 to 10 separates infertile eggs from early embryonic death. Clear eggs point back to the breeder flock, not to the setter.
- Early dead embryos. Concentrated in the first week, they suggest storage conditions, rough handling or a turning failure.
- Late dead embryos and failure to pip. Usually a water loss problem, sometimes ventilation. Weigh the sample tray before changing anything else.
- Contamination. Exploders and dirty eggs indicate hygiene at collection and in the machine. Never wash a hatching egg with cold water.
- Chick quality despite an acceptable hatch percentage. Frequently a temperature that ran slightly high during the second half.
A systematic break-out analysis of unhatched eggs, batch after batch, is the only method that converts these categories into decisions. Any suspicion of an infectious cause, whether egg-borne contamination or a breeder flock health issue, belongs with your poultry veterinarian rather than with a machine adjustment.
From hatch percentage to embryonic mortality
A hatch rate tells you how many chicks came out. Break-out data tells you why the others did not, and which stage of the process to change.
Sources: Petersime technical notes on eggshell temperature in setter and hatcher; Aviagen and Cobb hatchery documentation on measuring egg water loss; Lohmann Breeders on short periods of incubation during egg storage (SPIDES) and on pre-incubation of stored eggs; technical reviews of egg turning frequency and angle during incubation. Consulted August 2026.
Published previously, fully revised on 16 August 2026. General operational guidance for poultry professionals. It does not replace the assessment of the veterinarian responsible for your flock or the incubation guide published for your genetics.