Pipping in poultry is triggered by rising carbon dioxide levels inside the egg, which signal the chick to break through the shell membrane and then the shell itself. The process happens in two stages: an internal pip, where the chick pushes into the air cell at the blunt end of the egg, and an external pip, where it cracks through the outer shell. The time between these two events is generally 12 to 36 hours.
How Pipping Works Inside the Egg
In the final days of incubation, the chick rotates into hatching position: head tucked under its right wing, beak pointed toward the air cell at the wide end of the egg. The first breakthrough is the internal pip, when the chick pushes through the inner membrane and into that air pocket. For the first time, the chick begins breathing air rather than relying entirely on gas exchange through the shell’s pores.
But even though the eggshell is porous, carbon dioxide builds up in the air cell faster than it can escape. That rising CO2 concentration is the direct trigger for external pipping. The chick uses a small, hard projection on the tip of its upper beak called an egg tooth to punch through the shell from the inside. A specialized muscle at the back of the chick’s neck, sometimes called the pipping muscle or hatching muscle, powers this repeated striking motion. The exact mechanism by which CO2 drives this response is still not fully understood, but the relationship between gas buildup and the onset of pipping is well established.
Shell Structure and Gas Exchange
All gas exchange between the developing embryo and the outside world happens by diffusion through thousands of microscopic pores in the eggshell. The density of these pores, and therefore how easily gases move in and out, varies across different regions of the shell. More pores mean higher gas conductance, and this relationship is nearly proportional: a 35% increase in pore density produces roughly a 35 to 42% increase in gas flow.
Toward the end of incubation, the embryo pulls calcium from the inner surface of the shell to build its skeleton. This thins the shell slightly, which you might expect would make pipping easier. However, research on duck eggs shows that shell thinning alone doesn’t significantly change gas conductance. What matters more is pore density. The air cell at the blunt end of the egg actually blocks calcium mobilization in that region, because it disrupts the liquid connection between the inner and outer shell membranes. So the shell over the air cell, right where the chick pips, stays relatively intact.
For the chick, shell thickness and hardness directly affect how difficult external pipping is. Excessively thick or poorly porous shells can trap CO2 at dangerous levels or make it physically harder for the chick to break free. Eggs from older hens, or from flocks with certain mineral imbalances in their diet, can produce shells that are either too thick or too thin for ideal hatching.
How Temperature Affects Pipping Timing
Temperature is the single biggest factor controlling when pipping occurs. For forced-air incubators, the target is 99 to 99.5°F. Running consistently above that, even by half a degree or so, accelerates development and produces an early hatch. Running below it slows everything down and delays pipping. In a still-air incubator, where temperature is measured at the top of the eggs, the target runs slightly higher because heat stratifies in the chamber.
Temperature fluctuations during incubation are more damaging than a steady temperature that’s slightly off target. Repeated spikes and dips can confuse embryonic development, leading to chicks that reach pipping age but lack the strength or coordination to break through the shell. Prolonged overheating above 104°F is typically fatal, while extended cold spells slow metabolism enough that the embryo may never reach the pipping stage at all.
The Role of Humidity and the Air Cell
Humidity controls how much moisture evaporates through the shell over the 21-day incubation period (for chickens). That moisture loss is what creates the air cell at the blunt end of the egg, and the air cell is what the chick pips into first. If humidity is too high during incubation, the egg loses too little water, the air cell stays small, and the chick may drown or suffocate before it can pip into a large enough air pocket. If humidity is too low, the egg loses too much water, the membranes dry out and become tough, and the chick can become stuck or too dehydrated to hatch.
The recommended relative humidity during the first 18 days of chicken egg incubation is around 60 to 65%. During the final three days, when pipping and hatching occur, humidity should rise to 65 to 75%. This higher humidity during lockdown prevents the inner membrane from drying into a leathery seal around the chick once the shell is cracked. Breeders who open the incubator frequently during this stage let moisture escape and risk exactly that problem, sometimes called “shrink-wrapping.”
When the Chick Is in the Wrong Position
Positioning inside the egg is critical in the final 24 to 48 hours before hatch. The normal hatching position is head under the right wing, with the beak pointed toward the lower slope of the air cell. From this orientation, the chick can pip into the air cell, then rotate in a circle while chipping away at the shell (a process called “zipping”) until the cap pops off.
Six recognized malpositions can prevent successful pipping:
- Head over the right wing: generally not lethal, and many chicks in this position still hatch
- Head under the left wing: causes moderate to high mortality because the beak is angled away from the air cell
- Rotated away from the air cell: moderate to high mortality, since the chick pips into the wrong part of the egg or not at all
- Leg over the head: high mortality, as the leg physically blocks the beak from reaching the shell
- Upside down (head at the small end): high mortality, because the chick cannot reach the air cell to begin breathing
- Head between the legs: high mortality, with the chick completely unable to orient its beak toward the shell
These malpositions are lethal specifically because they prevent pipping or hatching. An embryo found in an abnormal position that died several days before the expected hatch date likely died from a different cause, such as infection or nutritional deficiency, rather than the malposition itself. Malpositions are more common when eggs aren’t turned frequently enough during the first 18 days, when eggs are set with the wrong end up, or when incubation temperatures are incorrect.
Nutritional and Genetic Factors
The chick’s ability to pip depends on having enough energy reserves and physical development to power the hatching muscle. Breeding hens that are deficient in key nutrients, particularly certain B vitamins, vitamin E, and selenium, produce embryos that develop normally through most of incubation but lack the vigor to complete pipping. These chicks often make an internal pip but never break through the shell, dying fully formed inside the egg.
Genetics also play a role. Some breeding lines produce consistently thicker shells or larger chicks relative to egg size, both of which can complicate pipping. Inbreeding in small flocks can reduce hatchability over generations, producing embryos that are more prone to malposition or weakness at hatch. Shell quality traits and hatchability are heritable, so persistent pipping problems in a flock sometimes point back to the breeding stock rather than the incubator.