Why Do Chickens Lay Unfertilized Eggs?

Chickens have been a part of farms and backyards for centuries now. Since humans have kept chickens, there has been one intriguing aspect about the species that continues to raise questions. Why do chickens lay unfertilized eggs? What is the purpose? Today, those questions are going to be answered as we delve into the biology behind egg development, the impact of selective breeding, and other factors that have given rise to chickens laying unfertilized eggs.

What to Know About Egg-Laying

Chickens are biologically predisposed to produce eggs. It is an integral part of avian reproduction. Interestingly, like human females, hens are born with a finite number of egg cells. They can’t stockpile them or even really choose when those egg cells are utilized. This is why hens tend to lay a large portion of their eggs as young adults then have their egg production taper off the older they get.

Yet, this limited supply of eggs has shaped how chickens go about attempting to bolster their population. Furthermore, much like many other species in the world, chickens and their reproductive cycle embrace something called “unpredictability.” Since hens can’t predict when a rooster will be available to fertilize their eggs, they give their species the greatest chance by having a more predictable egg-laying cycle.

Moreover, a hen’s reproductive cycle is governed by the pituitary gland, which is located beneath the skull. The phenomenon is also light-dependent (more on that in a bit). In order for the egg-laying cycle to initiate, a chicken must have 14-16 hours of daylight. This is why many chickens stop laying eggs during the winter when there is less daylight.

The Egg-Laying Cycle

To better understand how the hen’s cycle works, let’s take a look at each of the stages of egg development:

Ovarian Follicular Development (Variable Duration)

The cycle initiates with the growth and maturation of ovarian follicles in the hen’s ovaries. This phase can span several days to weeks, during which multiple follicles develop. Among these, one follicle becomes dominant and eventually releases the egg.

Ovulation (15-30 Minutes)

Once the dominant follicle matures, it ruptures, releasing the yolk into the oviduct. This process, known as ovulation, typically takes around 15 to 30 minutes. The yolk then begins its journey through the oviduct.

Formation of Egg White and Shell Membranes (3-4 Hours)

As the egg moves through the oviduct, the egg white (albumen) is secreted around the yolk. This process usually takes around 3 to 4 hours. Subsequently, the shell membranes are added to the forming egg.

Shell Formation (Around 20 Hours)

The egg progresses through the oviduct, and the shell is formed around it. This phase involves the gradual deposition of calcium carbonate layers onto the egg. It typically takes around 20 hours for the shell to fully form.

Egg Laying (About 5 Minutes)

Once the egg is fully developed, it is laid by the hen. The egg travels from the oviduct to the cloaca, and muscular contractions help propel the egg out of the hen’s body. The actual egg-laying process usually lasts around 5 minutes.

You can see a visualization of this process in this video:

The Natural Outcome

As we explore the question of why chickens lay unfertilized eggs, it’s essential to acknowledge that this behavior is a natural outcome of their reproductive strategy. Chickens lay eggs with the hope of fertilization, but they cannot predict whether a specific egg will be fertilized or not. In the wild, where mating among fowl is common, most eggs end up being fertilized.

Keep in mind that roosters do not operate on the same cycle as the hen. They will often court and mate any female that is receptive to them. The hen does not need an egg ready for fertilization in order for her to couple with a male. Sperm can remain viable inside a hen for several days, so there is a chance that any egg realized within that window will end up fertilized. Yet, there is no synchronization between a rooster’s mating behavior and a hen’s egg laying.

Now, take the same situation but adapt it to a commercial scenario, where hens are kept without roosters. Due to their biological programming, the hens are laying eggs as they normally would. What seems strange to people is actually very natural for the chicken. They are laying eggs to ensure the survival of their species, and so laying more than one egg, regardless of fertilization, is just part of that.

How Breeding Has Led to Prolific Egg-Laying

If humans had never discovered the benefits of eggs, would chickens have developed their ability to lay a prolific number of unfertilized eggs? Probably not. Through a process known as selective breeding or artificial selection, humans have significantly altered the behavior and genetics of chickens to serve our specific needs and desires.

Selective breeding involves intentionally choosing individuals with desirable traits and allowing them to reproduce, thereby passing those traits on to the next generation. In the case of chickens, humans selectively bred hens that exhibited higher egg-laying rates. Over generations, this practice resulted in the development of breeds that lay an impressive number of eggs, even without the presence of a rooster.

Seasonality and Lighting

Nature equipped the ancestors of the domestic chicken with a mechanism that allows them to adjust their egg-laying based on the season. In other words, chickens that have had less selective breeding will lay unfertilized eggs less often during cooler months when light is less abundant. This is also because chicks need ideal environment conditions to survive.

However, humans introduced changes to this normal cycle with, first, selective breeding, and secondly, artificial lighting. By providing hens with extended daylight hours, regardless of the actual season, humans can trigger their reproductive systems to lay eggs consistently. This practice is particularly important in commercial egg production, where consistent egg supply is essential for meeting consumer demands.

The Difference Between Fertilized and Unfertilized Eggs

Now that you know why chickens lay unfertilized eggs, you may be wondering if the ones in your basket are all unfertilized. Let’s take a look at the distinctions so you can spot when an egg has been fertilized or not:

Development

The most critical distinction is development. An unfertilized egg is never going to evolve into a fluffy chick, because it does not have the ingredients for life. Even if a hen sits on an unfertilized egg, it will not hatch. To compare, a fertilized egg will have the makings of life when certain conditions are met. Aside from being fertilized by a rooster, the egg will also start to develop when it is exposed to 100 degrees F and 60% humidity for several hours.

Fascinatingly, if you put a fertilized egg in the refrigerator, all further developmental progress is halted indefinitely. This proves the delicate nature of development and how external factors play a key role in hatching chicks.

Around 3 days of incubation, the egg will start to show some signs of fertilization, such as visible veining. However, other signs of development are very subtle and difficult for the naked eye to perceive.

A Small White Dot

There is a myth that you can tell if an egg is fertilized by the presence of a white dot known as a blastodisc. Unfortunately, you won’t be able to detect the presence of this disk without cracking the egg open and looking at the yolk. Both unfertilized and fertilized eggs have a blastodisc. Only fertilized eggs will gain a white dot that looks more like a bull’s eye, due to the growth of a ring called a blastoderm.

Red Spots and Chalaza

Eggs with red spots and chalaza are also common sights that are unrelated to fertilization and development. A red spot is a broken blood vessel. Similarly, the presence of white stringy “things,” often mistaken for an umbilical cord, is nothing more than a chalaza. The chalaza serves to suspend the yolk within the egg, preventing collisions with the shell.

Final Thoughts on Chickens Laying Unfertilized Eggs

Why do chickens lay unfertilized eggs? Hens will lay unfertilized eggs because the eggs develop before they are fertilized. This is part of survival for birds of all kinds, including chickens. Since the eggs are nearly formed prior to fertilization, they will be laid whether or not a hen couples with a rooster. Hopefully, this has helped you gain some insight into the world of chickens and their unique biology!