From a practical perspective, knowing how the hen makes her eggs goes a long way toward recognizing and understanding problems that our hens may have someday with their reproductive system. When hens are just starting to lay eggs and when they are much older, hens can experience laying problems such as shell-less eggs, being egg bound and double yolkers. Therefore, the goal of this article is to cover the basic reproductive tract of a female chicken and link that to how table eggs (aka the eggs that we eat) and fertilized eggs (aka the eggs that eventually hatch into a baby chick) are made in order to help you understand how some of these problems can be managed.


The Ovary of the Hen: Where It All Starts

Astute readers will notice that we wrote ovary and not ovaries. Unlike most animals that have 2 ovaries and oviducts (the tube attached to the ovary), birds only have a left ovary and left oviduct. While no one “knows” why this is so, the majority of adaptations that make birds, birds (feathers, hollow bones and lack of teeth) are focused on making birds light to make flying energetically easier. In other words, one fully developed ovary is lighter than two fully developed ovaries.


The fully developed left ovary is a cluster of sacs much like a bunch of grapes (Figure 1). Under the right conditions, each of these ova can develop into a table egg for eating or a fertilized egg that can hatch into a chick. Specifically, once a cute baby chick becomes an adult laying hen around 18-20 weeks of age a few yolks at a time increase in size (Figure 1). Once they get big enough, one ova or yolk is released from the ovary and goes on its magical journey through the oviduct.


Image of a chicken reproductive tract laying on a white benchtop in a "U" shape. On the left tip of the "U", the ovary and yellow ova of various sizes is shows. Progressing down the "U", the flesh colored infundibulum is shown connecting the ovary and magnum. On the right side of the trough of the "U", the isthmus is shown leading to the larger diameter shell gland/uterus, which finally leads to the vagina and vent at the top right of the "U". A white egg is shown next to the vent.

Figure 1. Reproductive tract of a female hen. The hen is born with all the ova she will ever have in the ovary. When she becomes sexually mature around 18-20 weeks, some yolks will grow and eventually either become eggs for eating or become fertilized by semen and under the right conditions develop into an embryo and hatch into a chick. Image by Dr. Jacquie Jacob, University of Kentucky.


Table Eggs

As noted above, once the yolk is large enough, it is released from the ovary via a process called ovulation. The released yolk is then picked up by the part of the oviduct closest to the ovary called the infundibulum (or funnel) (Figure 1). The muscular infundibulum grabs the released yolk and holds the yolk for around 15 minutes.  The yolk then passes to the magnum where the thick egg white (or albumin) is added around the yolk for the next 3 hours. The yolk and white now travel to the isthmus where the inner and outer shell membranes form for approximately one hour. Next, the egg goes to the shell gland (or uterus) where the shell forms around the egg. This process takes close to 20 hours and utilizes up to 10% of all the Calcium in the hen’s body! The shell gland is also where the pigment is added that makes the eggs blue, brown, speckled etc. Finally, the last part of the oviduct is the vagina which doesn’t play a part in egg formation but does two important things:

  • The bloom or cuticle forms on the shell in the vagina. The cuticle has some anti-bacterial properties which help mitigate (not eliminate) bacterial growth.
  • The vagina helps push the egg out of the body in such a way as to prevent fecal material from getting on the egg shell. This is a pretty amazing trick since the cloacca is also the orfice that poop leaves from!


Fertilization of the Egg

First the obvious…For a hen to lay a fertilized egg, she needs a rooster…


Mating is a funny and short process (takes just a few seconds) for chickens… The rooster circles the hen and makes a big “hub bub” and eventually mount the hen and bites down on the feathers of the hen’s head and neck for balance. Eventually they have sex. Specifically, the roosters papilla (which is the equivalent of a penis and is in their cloaca) delivers the sperm to the hen cloacca and away the sperm swims. IF fertilization is to occur, the sperm will fertilize the yolk in the infundibulum. Since the yolk is only in the infundibulum for around 15 minutes, hens have “sperm host glands” where sperm can be stored for up to 2 weeks! Therefore, stored sperm can be delivered to the yolk in order to allow for fertilization even if the rooster has delivered sperm when there is no yolk in the infundibulum!


The rest of the journey for the fertilized egg is identical to the table egg described above.  However, once the fertilized egg is laid, the egg will not develop until certain conditions are met. The most important being heat hence the hen will brood their eggs (fertilized or not). An incubator simulates that process.


Irregularities in egg development can occur with both table eggs and fertilized eggs. For example, occasionally a hen lays a shell less egg. In this case the egg bypasses the shell gland and or the shell is not completely deposited. This could be the result of a nutrition problem (not enough Calcium), infectious diseases such as Infectious Bronchitis virus, stress or simply due to age (young hens and old hens).

Egg yolk peritonitis is a syndrome where the infundibulum doesn’t catch the yolk and the yolk ends up in the abdomen. When the yolk is in the abdomen, inflammation and infection can occur. Unfortunately, this is a life-threatening problem (even with treatment) primarily due to the resulting infection. The best way to prevent EYP is via good management and nutrition.


In summary, knowing just a little about how eggs (fertilized and table eggs) are made gives us more useful information on how best to care for our chickens. With proper nutrition, biosecurity and husbandry we can help ensure that our chickens have a happy and healthy reproductive life-span.