Why Don’t All Eggs Become Chickens?

Most eggs never become chickens for the simplest possible reason: they were never fertilized. The eggs sold in grocery stores come from hens housed without roosters, so there is no embryo inside to develop in the first place. But even when an egg is fertilized, the journey from a single cell to a fluffy chick is a 21-day obstacle course where temperature, humidity, oxygen supply, shell quality, infections, and the hen’s own health all have to line up. Failure at any stage means the embryo dies quietly inside the shell.

The Grocery Store Egg Has No Embryo at All

A hen lays eggs whether or not she has mated with a rooster, much the way a human ovulates on a regular cycle regardless of fertilization. Commercial laying operations almost never keep roosters in the flock, because the goal is table eggs, not chicks. So the vast majority of eggs people eat contain only an unfertilized yolk cell. No amount of warmth or time could coax a chicken out of one of these eggs. The confusion is understandable: the egg looks the same either way. A fertile egg has a tiny disc of cells called a blastoderm on the yolk surface, but it is barely visible to the naked eye, and it does not change the taste or nutritional profile of the egg.

Cool Temperatures Put Development on Pause

When a fertilized egg is laid, the embryo is already a small cluster of cells. But chicken embryonic development requires sustained warmth near 37.5–37.8 °C (about 99.5–100 °F). If the egg cools below roughly 27 °C (80 °F) after laying, the embryo enters a state of dormancy where cell activity slows to a crawl and development effectively stops. This temperature-induced pause is how a hen can lay a clutch of eggs over several days and then begin incubating them all at once, so the chicks hatch around the same time.

Refrigeration pushes this pause even further. Store-bought eggs in the United States are typically held at around 4 °C (40 °F), which halts any cellular activity completely. Even fertile eggs that somehow ended up in a refrigerator would not develop, because the embryo has entered dormancy. The catch is that this pause is not indefinitely reversible. Research using imaging technologies found that storing eggs for ten days before incubation already impaired embryo growth, with measurable reductions in brain and embryo volume by the eleventh day of incubation.¹ Prolonged storage triggers cell-death pathways in the blastoderm, meaning the embryo’s viability gradually erodes even before anyone tries to warm it up. A field study of commercial hatcheries confirmed that storage time was the single most important factor affecting early embryonic mortality.²

Getting the Incubator Wrong

Assuming an egg is fertile and fresh, the next hurdle is the incubation environment itself. In nature, a broody hen manages this instinctively, adjusting her position, turning the eggs, and leaving the nest briefly for food and water. In artificial incubators, humans have to replicate those conditions, and small deviations can be lethal.

Temperature is the most critical variable. Research tracking chicken embryos under different incubation temperatures found that mortality was highest during the first four days when temperatures strayed from the ideal range, and that temperature-related disruptions produced more drastic effects on embryo survival than humidity changes did.³ Overheating is especially dangerous. Even a few degrees above optimal can cause malformations of the head, limbs, and abdominal wall in developing embryos, often killing them outright.

Humidity matters too, though its effects are more subtle. The egg needs to lose a specific amount of water through its shell over the 21-day incubation period. If the surrounding air is too dry, the egg loses water too fast, which can dehydrate the embryo; if the air is too humid, the egg retains too much water, potentially drowning the chick before it can pip through the shell. Studies have shown that low humidity increased late-stage embryonic mortality by about three percentage points and reduced the hatch rate of fertile eggs by a similar margin.⁴ The relationship is not perfectly linear, though. Research on broiler eggs found that lower humidity actually helped younger flocks by improving oxygen flow through the shell, while the same conditions hurt larger eggs from older flocks.⁵ There is no universal humidity setting that works perfectly for every egg.

Breathing Through a Shell

A chicken embryo cannot breathe in the way you or I do, but it absolutely needs oxygen and has to get rid of carbon dioxide. It accomplishes this through thousands of microscopic pores in the eggshell. These pores allow gas exchange with the outside air, and the conductance of these pores is remarkably well matched to the embryo’s metabolic needs. Research across multiple bird species found that oxygen consumption, carbon dioxide output, and water vapor loss per pore are all tuned to the embryo’s growth rate, with each individual pore handling roughly similar gas volumes regardless of species.⁶

When something disrupts that gas exchange, the embryo suffocates. This can happen if the shell is unusually thick (fewer effective pores per unit area), if the shell membrane becomes blocked by moisture or contamination, or if the ambient air itself is low in oxygen. High-altitude incubation illustrates this starkly: at 2,900 meters above sea level, the thinner air provides less oxygen per breath of gas exchange, and researchers found that oxygen deficit was the main factor behind the poor hatchability and chick quality of lowland chicken breeds incubated at altitude.⁷ Breeds that evolved at high altitude fare much better, because their eggs and embryos are adapted to function with less available oxygen.

Bacteria and Other Invaders

An eggshell is a barrier, but not a perfect one. Those same pores that let oxygen in can also let microbes through. Eggshells harbor bacteria shortly after laying, and under warm, humid conditions, those bacteria can multiply rapidly, penetrate the shell, infect the egg contents, and kill the embryo.⁸ This is actually one reason incubation itself helps: the sustained warmth of a brooding hen tends to suppress microbial growth on the shell surface compared with an unattended egg sitting at ambient temperature.

In wild birds, the problem can be severe. A study of Arctic-nesting geese examined eggs that failed to hatch and found microbial infections throughout. When researchers isolated bacteria from these failed eggs and injected them into developing chicken eggs in the lab, mortality rates ranged from 70 to 100 percent depending on the bacterial species and dose.⁹ Commercial hatcheries spend considerable effort on sanitation, egg washing, and fumigation precisely because of this risk, but contamination still accounts for a meaningful share of embryo losses.

What the Hen Eats Matters More Than You’d Think

The embryo inside the egg has no external food source. Everything it needs for 21 days of development has to already be packed inside the egg when it is laid, and the hen is responsible for depositing all of it. That means her diet directly determines whether the embryo gets the right balance of nutrients. When a hen’s nutrition is deficient, excessive, or imbalanced, the effects on the embryo become more severe and tend to show up at earlier stages of development.¹⁰

Vitamin D is a good example. The embryo needs vitamin D metabolites to mobilize calcium from the inner surface of the eggshell, which it uses to build its skeleton. Research found that hens kept on a vitamin D-deficient diet laid eggs with perfectly fine shell quality on the outside, but their embryos could not access the calcium locked in that shell and died before hatching.¹¹ The shell looked great; the embryo just could not use it.

Contaminants in the feed pose a separate threat. Mycotoxins, which are toxic compounds produced by mold in grain, can pass from the hen’s feed into the egg. Corn contaminated with aflatoxin, deoxynivalenol, and zearalenone has been shown to decrease hatchability and increase late-stage embryo mortality in broiler breeders.¹² Interestingly, the impact varies by breed: one study found that a specific mycotoxin reduced fertility and hatchability in one breed of laying hen but actually had no negative effect, or even a slight positive one, in a different breed.¹³ Genetics shapes how vulnerable a given bird is to these dietary poisons.

The Age of the Hen and the Quality of the Shell

Older hens tend to lay eggs that are less likely to produce chicks. Fertility declines with flock age, and studies of broiler breeders have found statistically significant drops in both fertility and hatchability as hens get older.¹⁴ Part of this is straightforward reproductive aging: fewer eggs are successfully fertilized. But the eggs themselves also change. Older hens lay larger eggs, often with thinner shells, which alters gas exchange and makes the eggs more vulnerable to physical damage and microbial penetration.

Shell thickness turns out to be a surprisingly important predictor of hatching success, especially when eggs have to be stored before incubation. In one study, eggs with thick shells from an older flock hatched at about 58 percent, while the overall hatchability for that same older flock’s eggs was only 38 percent.¹⁵ Thick-shelled eggs simply tolerated the stress of storage better. The shell is not just packaging; it is an active part of the life-support system.

Fertility decline with age has some nuance. While one large study confirmed that older hens have lower fertility and hatchability overall, a separate study looking at the same question found no difference in hatchability of fertile eggs or embryonic mortality when comparing young and old flocks.¹⁶ The distinction matters: older hens lay fewer fertile eggs, but once an egg from an older hen is actually fertilized, it may have a comparable chance of hatching. The bottleneck is more about fertilization than about embryo survival, at least in some breeds.

What Happens in Wild Nests

Everything discussed so far applies to commercial poultry, where humans control the environment. Wild birds face all the same challenges plus several more: predation, weather extremes, nest abandonment, and simply being away from the eggs for too long. A study of tropical birds tested what happens when eggs sit unincubated in warm climates. Control eggs that were incubated normally hatched at about 82 percent. Eggs left exposed for just one day still hatched at roughly 79 percent. But after three days without incubation, hatching success plunged to about 42 percent, and after seven days it was barely 2 percent.¹ In a hot climate, the egg does not stay safely dormant the way it would in a cooler environment; temperatures may be warm enough to trigger partial development but not warm enough to sustain it, creating a lethal middle ground.

Wild bird eggs also face bacterial challenges that commercial eggs do not, as the Arctic goose study demonstrated. Nest sanitation is nonexistent, eggs sit on soil or vegetation teeming with microbes, and the parent’s absence for foraging exposes the eggs to temperature swings that can promote microbial growth on the shell surface. For many bird species in the wild, a hatching rate below 80 percent is perfectly normal, and a significant fraction of eggs never produce offspring even under ideal conditions.

When an Unfertilized Egg Tries Anyway

There is one genuinely strange exception to the rule that an unfertilized egg cannot develop: parthenogenesis. This is a process where an egg begins dividing without being fertilized by sperm. It happens naturally in some insect and reptile species, but it also occurs in birds, particularly turkeys. The phenomenon is rare and almost always fails. A review of the research describes avian parthenogenesis as “mostly abortive in nature,” with delayed, disorganized development and hostile conditions inside the unfertilized egg that typically halt progress well before anything resembling a viable embryo forms.¹⁷

In turkeys, parthenogenetic embryos are always male, a consequence of the way bird sex chromosomes work. A study that genetically sexed 35 parthenogenetic turkey embryos found every single one was male, whereas a comparison group of normally developing embryos included females as expected.¹⁸ While a handful of cases have been documented where a parthenogenetic turkey embryo survived to hatching, these are extreme outliers. For chickens, the phenomenon is even rarer and has essentially no practical significance. It is more of a biological curiosity that reminds us development can be triggered by mechanisms other than normal fertilization, even if it almost never succeeds.

Why Commercial Hatcheries Still Lose Eggs

Given all the factors that can go wrong, you might wonder what kind of success rate a well-run commercial hatchery actually achieves. The answer is typically somewhere around 80 to 90 percent of fertile eggs, depending on the breed, flock age, and management practices. That means even in optimized conditions, roughly one in every five to ten fertile eggs does not hatch. The losses are distributed across the entire incubation period but tend to cluster in two windows: the first few days (early embryonic mortality, often caused by storage damage, genetic defects, or temperature shocks) and the last few days (late mortality, when the chick is transitioning to lung breathing and has to pip through the shell).

Hatcheries invest heavily in environmental controls, egg selection, breeder nutrition, and sanitation to push that number as high as possible, because every unhatched egg represents a direct economic loss. But biology is messy. Some fraction of embryos carry lethal genetic mutations that halt development at specific stages. Some eggs have invisible hairline cracks that allow infection. Some embryos are simply positioned wrong and cannot orient themselves to pip the shell. No technology eliminates every source of failure.

For backyard chicken keepers working with a small incubator and eggs from their own flock, hatch rates can vary wildly, from over 90 percent in ideal circumstances down to 50 percent or lower when storage, temperature control, or humidity management is off. The most common mistakes beginners make are storing eggs too long before setting them, running the incubator at the wrong temperature, and failing to turn the eggs frequently enough during the first 18 days. Each of these errors maps directly to the biological mechanisms described above: cell death during prolonged dormancy, heat-induced malformations, and the embryo adhering to the shell membrane when it is not rotated.

How Altitude Changes the Equation

People raising chickens at high elevations face a challenge that lowland farmers never think about. The reduced oxygen pressure at altitude starves the embryo of the gas it needs to fuel its metabolism. Research comparing lowland chicken breeds incubated at 2,900 meters found that oxygen deficit was the primary culprit behind poor hatch rates and weaker chicks.⁷ Breeds native to high-altitude regions, such as those raised for centuries in the Andes or the Tibetan Plateau, have adapted over many generations with higher shell porosity and embryos that tolerate lower oxygen levels. For anyone trying to hatch eggs above about 1,500 meters, choosing breeds adapted to altitude or supplementing oxygen in the incubator can make a meaningful difference in how many eggs actually produce chicks.