Friday, January 18, 2013

Chickens, Eggs, Fertilization, Reproduction and Other Insider Information

One of my daughters has a Silkie that pops out eggs like a gumball machine! She layes almost daily... about every 25 hours. Ever wonder how chickens do that?

On this blog, it is not that uncommon for me to show the insides of real chickens. Happily, I can assure you that through the magic of electronics and art, my daughter's gumball machine Silkie (today's model of chicken reproduction) is running happily about the yard as I write.

So in this chicken version of How Stuff Works, I have made a rather simple diagram of the inner reproductive workings of the average hen. Chickens develop and hatch with thousands of ova (yolks or undeveloped eggs) - already inside their ovary. Though chickens start out (in the egg) with two ovaries, only the left one develops and becomes active. It is this way with almost all poultry, though raptors differ in that generally the right side develops and not the left.

In this diagram you can see Ova in various stages of development near the #1.

As a pullet matures, her reproductive organs mature also. From the outside we can usually see the growth and reddening of the comb and wattles... unless, that is, it is a Silkie where comb and wattles tend to be dark in color and hidden under tons of fluff.

When a mature Ovum is released from the Ovary, it is called ovulation. The entire reproductive system is called the Oviduct and  is usually between 25 and 27 inches long in a mature hen. The Oviduct has five distinctive parts. The first part is the Infundibulum - When the ovum is released from the ovary, the muscle lining of the infundibulum pulls the ovum into it... sort of like a mini egg vacuum. The ovum spends about 15 to 18 minutes in this part of the oviduct.

Fascinating fact - when a rooster mates with a hen, he mounts her and, standing on her back, lowers his cloaca (vent) and the hen inverts her own cloaca to meet with his. There is no penetration, but the sperm packet released by the male is taken into the hen's cloaca or vent. From there the sperm makes its way to the infundibulum where it awaits the release of an ovum. Sperm can live in the infundibulum for more than 2 weeks.

So - the ovum is released from the ovary into the 3 to 4 inch-long infundibulum where it is fertilized with the rooster's sperm if the hen has been mated.

 Then the fertilized (or unfertilized) ovum moves into the Magnum. The magnum is the longest section of the oviduct at roughly 13 inches in length (diagram is SO not to scale). This is where the egg white or Albumen is added to the ovum. The ovum's trip down the magnum takes about 3 hours.

Next, the partially-constructed egg moves into the 4 inch long Isthmus where the inner and outer shell membranes are added over a period of about 75 minutes.

The egg then moves into the Shell Gland (Uterus) where it stays for 20 hours to have its shell constructed. The hen's body will pull some calcium from her bones and the rest from her diet to put into the shell. If the hen is of a breed that lays colored eggs, the pigment will also be put into the egg in this section of the oviduct.

After the shell has been added, the egg moves down toward the Cloaca or Vent so that the chicken may lay the egg.

And from here the entire process starts over. Interesting facts to note, chickens almost never lay eggs after dark. It takes an average of 25 hours to form a new egg. This means that even the best layers will have to skip a day every now and then because her egg comes an hour later each day, and once she reaches point at dusk, her body will hold on to the egg until the next morning.

Often people wonder about pullets laying shell-less eggs or irregular eggs. The best way to explain this is that as a pullet matures, all those parts of the oviduct may not be ready to create an egg all at the same time. If the Shell Gland is not ready or hasn't been able to procure enough calcium from the hen's body or diet, it may not be able to form a shell properly. It is best not to eat shell-less eggs. Feed them to your chickens - they love eggs for breakfast!

Hens that seem to have a habit of laying eggs with ridged or rough shells likely have an abnormality in their Shell Gland. Generally it is harmless to the hen, and perfectly safe to eat eggs with ridged shells.

Sometimes small spots of blood or meaty substances are present in eggs. This is caused when some part of the oviduct sheds material or has a broken blood vessel, and the shed matter simply gets wrapped up inside the egg when the membranes or shell is added. Though it should be fine to eat such eggs (fully cooked), that is solely up to you.

It is also very common for pullets to have some funny issues with laying when they have just started, and it is also common for mature hens to have some issues when they re-start laying after being broody or after a molt.

Oh - and one last thing... while feces and eggs both exit through the Vent, the oviduct and the end of the large intestine open right before the inside of the vent opening. This is why it is very, very uncommon to have worms inside an egg... the oviduct is placed far from the intestinal tract where parasites live to  help prevent such things. People may mistake a bit of sloughed off oviduct lining for a parasite - - because any parasite that found its way into an egg would have had to take a seriously wrong turn somewhere along the way!! Of course, it is not impossible. If it truly is a parasite inside one of your hen's eggs, it could indicate an uncommonly high parasite load in that chicken, and it would be time to take action to remedy the situation.

Happy Laying!



  1. Wow...done incredibly well! Good illustrations. Good information. And a good base photo! Definitely explained and visually illustrated in such a way that it's very easy to see and understand.


  2. Such a wonderful job! Your illustrations are top notch.

    I would like to add:
    worms in eggs are rare because of the construction of the hen
    but, you still can have worms in your eggs if your flock is infested.


  3. GREAT info... I've been curious about exactly how eggs are formed, and the time line of all the different development phases they go thru as they travel thru the oviduct from start to finish. Now I know :) Thanks for blogging this!

  4. This would be a great illustration for elementary school students science class!!! Well done.

  5. As always, very easy to follow and understand. Beautiful graphics. Excellent information.

  6. It all makes sense to me now!! Thanks for sharing!

  7. I wish you would blog about sexing chickens. We found out last weekend that one of our pullets is actually a rooster. "She" crowed. Not just once, but six times in succession. We got the "pullets" from a poultry farmer that assured us they were all pullets. I now wonder if he was wrong about any of the other "girls". We have Isa Reds. Can you help me know what to look for?

    1. Lorri -
      Unfortunately even the best pros at sexing chicks have only about a 70% accuracy rate. The reason for this is that the sex organs of chickens are not only internal, but also very, very tiny. Added to that, male organs can look like female organs until the hormones kick in in some chickens.

      If you want to avoid "mistakes" in the future, one of the only ways to do this is to buy auto-sexing or sex-linked breeds (Barred Rocks, Cream Legbars, Red Sexlinks and Black Sexlinks). In these breeds the males and females are marked differently at hatch.

      Please know this mistake was just that. A mistake. I don't believe anyone was trying to fool you. This happens all the world over to many, many chicken keepers.

  8. I had always wandered how Hens reproduce with a Cock without a cock... Now I know.. Cheers

  9. great explanation of the egg laying process. I have 7 chickens and a bantam cock, which I acquired at the beginning of summer, my first time keeping chickens. Laying and eggs are great, but sometimes the eggs seem to take a long time to cook the white for a boiled egg, and this morning the egg yolk seemed to blend into the white (mind you the yolk was hard boiled). Is this because they are very fresh, but also are they different because of being fertilised?

    1. There can be a number of reasons your fresh eggs take longer to boil - the most likely of which is that healthy hens lay eggs with thicker, harder shells. This can make cook times a bit longer. As for the egg you had this morning, there may have been a hiccup when the hen was making the egg or possibly the yolk was broken inside the egg. The egg can become "scrambled" without being broken if it is shaken or tossed about a bit.

  10. Thank u very much. Nice work:-)

  11. At 57 yrs old, I never knew more than what my dear mom told me at 5 ... chickens just need 'a rooster' ... and as she had grown up on a farm, and I was young, trusting, and naïve, I believed that was all there was to it! Thanks for clearing it up so explicitly and with such great illustrations! Maybe she really didn't know herself as of the 16 kids in her family, the boys did the barns, coops, and crops while the girls did more 'female' pursuits. Times have changed for the better. A retired lawyer ...