How a Hen Makes an Egg & Egg Oddities.

In this article we'll look at the some of the most common strange eggs and their possible causes, but first we'll need to know how a hen’s reproductive system is supposed to work when firing on all cylinders.
Egg irregularities from backyard chickens are very common. The occasional strange-looking egg from a hen is to be expected. Take a picture of it, discuss it at the water cooler next day, but don’t worry too much about it- the vast majority of the time they do not indicate a serious problem. In this article we’ll look at the some of the most common strange eggs and their possible causes, but first we’ll need to know how a hen’s reproductive system is supposed to work when firing on all cylinders.

In this article we'll look at the some of the most common strange eggs and their possible causes, but first we'll need to know how a hen’s reproductive system is supposed to work when firing on all cylinders.Here’s the deal with a hen’s reproductive system: a female chick’s ovary contains all of the ova it will ever have when it’s hatched. When she is sexually mature, lighting stimulates hormones that cause a yolk to be released from the ovary into the oviduct where it acquires the albumen, (whites) membranes, shell and any shell color. The whole process takes approximately 25 hours to complete. Within about thirty minutes of an egg being laid, the process starts again.

 

 

A female chick's ovary contains all of the ova it will ever have when it's hatched. The ovary begins to convert ova to egg yolks when she is mature. With the right lighting conditions exists, hormones stimulate ova to develop into yolks. Yolks are released from the ovary into the oviduct when they reach the right size and travel down the oviduct to acquire their whites, membranes, shell and shell color, if any.

A hen's reproductive system consists of an ovary and oviduct (a long tube with several parts that have different jobs)
A hen’s reproductive system consists of one functioning ovary and oviduct (a long tube with several parts that have different jobs)

This is an actual hen’s reproductive tract*. I have labeled the functions that occur at different junctures along the way edited to add: if fertilization is to occur, it happens in the infundibulum,  which is the area immediately to the right of the ovary (the black line is running through it). The infundibulum is a muscle that essentially engulfs the ovum when it is released. The sperm waits in the infundibulum and has a narrow, 15-18 minute window of opportunity to fertilize the ova.

Egg irregularities from backyard chickens are very common. The occasional strange-looking egg from a hen is to be expected.

This is the ovary of a hen, showing ova (yolks) in various stages of development during a post mortem exam.
This is the ovary of a hen, showing ova (yolks) in various stages of development during a post mortem exam
Anatomy of an egg

ADVISORY: The following video contains anatomical images of a hen laying an egg. Viewer discretion is advised.

FERTILIZED & UNFERTILIZED EGGS

UNFERTILIZED EGG
A hen must mate with a rooster in order for her egg to contain both the male and female genetic material necessary to create an embryo inside the egg. An unfertilized egg does not contain the rooster’s genetic material, which means a chick can never hatch from that egg.  Every egg contains a concentration of cells containing the hen’s genetic material on the yolk. These cells are termed the blastodisk and they look like an irregularly shaped, white circle.
A hen must mate with a rooster in order for her egg to contain both the male and female genetic material necessary to create an embryo inside the egg. An infertile egg does not contain the rooster's genetic material, which means a chick can never hatch from that egg.FERTILIZED EGGS
When an egg is fertilized by a rooster, the blastodisk becomes known as the blastoderm, which is the first stage of embryo development. The blastodisk contains the genetic material from both the hen and the rooster. The blastoderm is also known as the germinal disc. When incubated under specific temperatures and humidity levels for 21 days, these cells can develop into a chick. The blastoderm is characterized by its bullseye appearance of regular, concentric circles.
Fertile egg identified by the blastoderm or germinal disc with regularly shaped, concentric circles, a bullseye appearance

EGG IRREGULARITIES

DOUBLE YOLKS
Commonly occur in new layers when the yolk release is mistimed and two yolks travel down the oviduct together. Some hens are genetically predisposed to laying double-yolked eggs.
DOUBLE YOLKS Commonly occur in new layers when the yolk release is mistimed and two yolks travel down the oviduct together. Some hens are genetically predisposed to laying double-yolked eggs.

This double yolked egg was laid by my Easter Egger, Esther.
This double yolked egg was laid by my Easter Egger, Esther

Can a double yolk egg hatch? Yes, but it’s rare. While extraordinarily uncommon, miraculously it can happen.

 

This double yolked egg weighed 90 grams. Yow.
This double yolked egg weighed 90 grams. Yow!
Triple yolker. Yikes.
Triple yolker. Yikes!

NO YOLK
Tiny eggs containing no yolk are referred to as fairy eggs, rooster eggs, wind eggs, dwarf eggs, rooster eggs or fart eggs (I don’t make this stuff up). These eggs are common in new layers when the reproductive system isn’t quite synchronized yet. They can also occur in older layers when a piece of tissue from the reproductive tract breaks free and tricks the hen’s reproductive system into treating the tissue like a yolk, creating an egg out of it. A little piece of tissue is visible in this photo:

Egg with no yolk, see tissue fragment at 3 o'clock in this picture.
Egg with no yolk, see tissue fragment at 3 o’clock in this picture

Wind egg, fairy egg, rooster egg, dwarf egg- learn how they occur here!
NO SHELL OR THIN SHELL
I call soft-shelled eggs rubber eggs because the membrane is soft and pliable. Commonly produced by new layers, caused by stress, an immature shell gland, a nutritional deficiency or a glitch in the uterus, aka: shell gland. To find them occasionally is no cause for concern, to find them regularly can indicate a calcium, phosphorous or vitamin D deficiency. High temperatures can also cause thin-shelled eggs due the hen’s decreased ability to store calcium in hot weather.

I call soft-shelled eggs rubber eggs because the membrane is soft and pliable. Commonly produced by new layers, caused by stress, an immature shell gland, a nutritional deficiency or a glitch in the uterus, aka: shell gland. To find them occasionally is no cause for concern, to find them regularly can indicate a calcium, phosphorous or vitamin D deficiency. High temperatures can also cause thin-shelled eggs due the hen's decreased ability to store calcium in hot weather.
These soft-shelled eggs feel like water balloons
These soft-shelled eggs feel like water balloons.
This egg was the first laid by my Easter Egger, Lucy
This was Lucy's second attempt. I found it in the run. It must have come as a surprise to her. Her eggs normalized after the first two.
This was Lucy’s second attempt. I found it in the run. It must have come as a surprise to her. Her eggs normalized after the first two.

Basket of colored eggsODD SHELL SHAPE OR TEXTURE
(Includes too large, too small, flat-sided, ‘body-checked’ eggs) I affectionately refer to these as ‘mutant eggs.’ In new layers, an immature shell gland can cause odd shell shape and is most often of no concern. In senior layers, oddly shaped eggs can result from stress or, if they are a regular occurrence, a defective shell gland. Misshapen eggs can also be caused by infectious bronchitis or egg drop syndrome, both of which are cause for concern.Shells with wrinkles or ‘checks’ in the shell are known as ‘body check’ eggs. These eggs have been damaged while in the shell gland from stress or pressure put upon them. The cracks in these eggs are repaired in the shell gland, resulting in checks or wrinkles.

Shells with wrinkles or ‘checks’ in the shell are known as ‘body check’ eggs. These eggs have been damaged while in the shell gland from stress or pressure put upon them. The cracks in these eggs are repaired in the shell gland, resulting in checks or wrinkles.ROUGH SHELLED OR PIMPLED EGGS
Egg shells can have different textures causes by a range of things from excess calcium or Vitamin D intake (pimpled eggs) to double-ovulation, disease, defective shell gland or rapid changes in lighting conditions (sandpaper eggs). As long as these types of eggs are found infrequently, there is no cause for concern.
Pimples may be caused by excess calcium intake. (You know how delicious those oyster shells can be!) Pimples on eggs can also be an indication of too much Vitamin D in a hen’s diet.

 

Egg shells can have different textures causes by a range of things from excess calcium intake (pimpled eggs) to double-ovulation, disease, defective shell gland or rapid changes in lighting conditions (sandpaper eggs). As long as these types of eggs are found infrequently, there is no cause for concern.
One sign of too much vitamin D in a hen’s diet is calcium ‘pimples’ on eggshells that, when scraped off, leave little holes in the shell

Pimples may be caused by excess calcium intake. (You know how delicious those oyster shells can be!) Pimples on eggs can also be an indication of too much Vitamin D in a hen's diet.FLAT SIDE – SLAB SIDED EGG
Can occur in new layers due to stress or disease. The egg is kept too long in the shell gland, resulting in a flat side with wrinkles. Can also occur when a mis-timed, second egg proceeds down the oviduct, bumping into and resting alongside the first egg.
Pimples may be caused by excess calcium intake. You know how delicious those oyster shells can be!

Flat egg.
Flat egg

LARGE EGGS
Eggs of unusually large size ordinarily contain double yolks and the hen’s reproductive system accommodates for the anomaly by working overtime to generate these monstrosities. Older hens tend to lay larger eggs.  On average, an extra large egg weighs 64 grams and a jumbo egg weighs 71 grams. The two largest eggs I’ve ever had were 90 and 95 grams.
Eggs of unusually large size ordinarily contain double yolks and the hen's reproductive system accommodates for the anomaly by working overtime to generate these monstrosities.

Eggs of unusually large size ordinarily contain double yolks and the hen's reproductive system accommodates for the anomaly by working overtime to generate these monstrosities.
This egg weighed 95 grams. The Marans eggs next to it were big eggs compared to the average egg.
This is the 95 gram egg next to an average Ameraucana egg.
This is the 95 gram egg next to an average Ameraucana egg

EGG-WITHIN-AN-EGG
An egg within another egg occurs when an egg that is almost ready to be laid reverses engines into the reproductive tract, meeting up with another egg-in-progress. It gets another layer of white/albumen and a new layer of shell before being laid. The cause is not known. While the literature characterizes this egg-within-an-egg phenomenon as “rare,” my sense is that it is significantly more common than previously believed. Many backyard chicken-keepers report discovering eggs-within-eggs from their hens.
An egg within another egg occurs when an egg that is almost ready to be laid reverses engines into the reproductive tract, meeting up with another egg-in-progress.An egg within another egg occurs when an egg that is almost ready to be laid reverses engines into the reproductive tract, meeting up with another egg-in-progress.
An egg within another egg occurs when an egg that is almost ready to be laid reverses engines into the reproductive tract, meeting up with another egg-in-progress.
BLOOD SPOT
A blood spot inside an egg can occur either as a result of a blood vessel breaking in the ovary when the yolk is being released or in the oviduct as the yolk travels through it. Blood spots may occur in older hens that have a genetic predisposition to them, in hens that have a vitamin A deficiency, or randomly in any egg.

Both a spot and a blastoderm are visible on this fertile egg.
Both a spot and a blastoderm are visible on this fertilized egg.

As opposed to blood spots, which occur on the surface of an egg yolk, meat spots are found in the egg white (albumen). Meat spots are the result of a small piece of the oviduct sloughing off as egg white is being added in the egg-making process.
MEAT SPOT
As opposed to blood spots, which occur on the surface of an egg yolk, meat spots are found in the egg white (albumen). Meat spots are the result of a small piece of the oviduct sloughing off as egg white is being added in the egg-making process. Meat spots can be removed with the tip of a knife and while visually unappealing, would be safe to eat.
Cause of meat spot inside a chicken egg

SHELL COLOR

All eggshells start out white. It is believed that colored eggs have their pigment added to the shell at the end of the egg formation process between the shell gland and vagina. White eggshells have no pigment at all.Brown eggshells contain the pigment protoporphyrin, ( a by-product of hemoglobin) which is found only on the surface of the shell. Brown pigment is applied during the formation of the last layer of the egg, the bloom or cuticle. The brown pigment can be rubbed off easily and does not color the inside of the shell.Brown
Eggshells contain the pigment protoporphyrin IX ( a by-product of hemoglobin) which is found only on the surface of the shell. Brown pigment is applied during the formation of the last layer of the egg, the bloom or cuticle. The brown pigment can be rubbed off easily and does not color the inside of the shell.
Brown eggshells contain the pigment protoporphyrin, ( a by-product of hemoglobin) which is found only on the surface of the shell. Brown pigment is applied during the formation of the last layer of the egg, the bloom or cuticle. The brown pigment can be rubbed off easily and does not color the inside of the shell.Brown eggshells contain the pigment protoporphyrin, ( a by-product of hemoglobin) which is found only on the surface of the shell. Brown pigment is applied during the formation of the last layer of the egg, the bloom or cuticle. The brown pigment can be rubbed off easily and does not color the inside of the shell.Brown eggshells contain the pigment protoporphyrin, ( a by-product of hemoglobin) which is found only on the surface of the shell. Brown pigment is applied during the formation of the last layer of the egg, the bloom or cuticle. The brown pigment can be rubbed off easily and does not color the inside of the shell.Blue
Eggshells are produced by the pigment oocyanin, (a by-product of bile formation). The color is applied early in the shell’s formation and penetrates the entire shell. The blue coloring cannot be rubbed off.
Blue eggshells are produced by the pigment oocyanin, (a by-product of bile formation). The color is applied early in the shell's formation and penetrates the entire shell. The blue coloring cannot be rubbed off.Blue eggshells are produced by the pigment oocyanin, (a by-product of bile formation). The color is applied early in the shell's formation and penetrates the entire shell. The blue coloring cannot be rubbed off.Green
Eggshells are a combination of blue and brown pigment being applied to the eggshell in the shell gland. The blue is added first and penetrates the entire egg while the brown pigment is laid on the surface of the eggshell.
Green eggshells are a combination of blue and brown pigment being applied to the eggshell in the shell gland. The blue is added first and penetrates the entire egg while the brown pigment is laid on the surface of the eggshell.Green eggshells are a combination of blue and brown pigment being applied to the eggshell in the shell gland. The blue is added first and penetrates the entire egg while the brown pigment is laid on the surface of the eggshell.
Uneven shell coloring results from the uneven distribution of pigment as the egg passes through the oviduct. Sometimes an egg is stalled for a time in the uterus, which allows more time for pigment to be applied.
Uneven shell coloring results from the uneven distribution of pigment as the egg passes through the oviduct. Sometimes an egg is stalled for a time in the uterus, which allows more time for pigment to be applied.Uneven shell coloring results from the uneven distribution of pigment as the egg passes through the oviduct. Sometimes an egg is stalled for a time in the uterus, which allows more time for pigment to be applied.

Uneven shell coloring results from the uneven distribution of pigment as the egg passes through the oviduct. Sometimes an egg is stalled for a time in the uterus, which allows more time for pigment to be applied.
This egg was laid by my neighbor’s hen during a heatwave. The temperatures were over 100 degrees most of the week, which is uncommon where we live. The stress of the heat is the likely cause of this unusual & spectacular coloring.
Spots on this brown egg appear purple due to the uneven distribution of brown pigment.
Spots on this brown egg appear purple due to the uneven distribution of brown pigment.
This camouflage egg was laid in the heat of the summer by my Easter Egger, Esther, who generally lays an khaki colored egg.
This camouflage egg was laid in the heat of the summer by my Easter Egger, Esther, who generally lays an khaki colored egg.
This egg was laid by one of my Blue Splash Marans. The colored flecks could be rubbed off very easily.
This egg was laid by one of my Blue Splash Marans. The colored flecks could be rubbed off very easily.
This spotted egg was laid by one of my Easter Eggers.
This spotted egg was laid by one of my Easter Eggers.
This egg was laid by one of my Olive Eggers.
This egg was laid by one of my Olive Eggers.

LASH EGG, aka: a PUS COAGULEGG
IS NOT AN EGG Lash eggs result from an infection (bacterial or viral) that causes inflammation of a hen’s oviduct. That inflammation is referred to as Salpingitis. The hen’s immune system reacts to the inflammation by trying to wall-off the infection with a waxy, cheese-like pus. This pus mass may or may not contain yolk, albumen, (egg white) eggshell, egg membrane, blood or pieces of tissue from the oviduct wall, but it is primarily a yellowish, cheesy,  pus ball. This is an ominous sign of a very serious infection that most hens do not recover from. Much more about lash eggs in this article.
Lash eggs consist primarily of coagulated pus, not yolk or egg white. I took the liberty of renaming the lash egg more appropriately, a Pus Coagulegg. It’s got a nice ring to it, don’t you think? Unfortunately for the hen, the Pus Coagulegg is no laughing matter as the prognosis for a bird producing them is poor, at best.Lash eggs result from an infection (bacterial or viral) that causes inflammation of a hen’s oviduct. That inflammation is referred to as Salpingitis. The hen’s immune system reacts to the inflammation by trying to wall-off the infection with a waxy, cheese-like pus. This pus mass may or may not contain yolk, albumen, (egg white) eggshell, egg membrane, blood or pieces of tissue from the oviduct wall, but it is primarily a yellowish, cheesy, pus ball. This is an ominous sign of a very serious infection that most hens do not recover from. Much more about lash eggs in this article.
GREEN EGG YOLK or PINK or GREEN EGG WHITES
“I do not like them Sam-I-Am. I do not like green yolks or ham!”
Feeding cottonseed meal to hens can cause green egg yolks and/or pink whites. Acorns in excess can have a similar effect. These eggs are perfectly edible, but may be visually off-putting. It is not surprising to me that I have not personally seen a a green egg yolk or found pink egg whites in an egg from my backyard chickens because I do not feed them cottonseed meal and we do not have any oak trees in our yard. Green egg whites can be caused by too much riboflavin in the hens’ diet.

Feeding cottonseed meal to hens can cause green egg yolks and/or pink whites. These eggs are perfectly edible, but may be visually off-putting.
This photo is a simulation of a green yolk. The bowl actually was green though. ☺

The preceding information is provided as a general guideline to understanding some egg irregularities and some of the more common causes of them. It is not intended as an exhaustive review of the subject. If you have some concern that your hen may be ill or if she consistently produces irregular eggs, you should consult an avian vet or perform in-depth research based upon your individual circumstances.
Whatever you are, be a good one. Good eggs.The Chicken Chick®
*Anatomical illustrations and photo reproduced for educational purposes, courtesy of  Jacquie Jacob, Tony Pescatore and Austin Cantor, University of Kentucky College of Agriculture. Copyright 2011. Educational programs of Kentucky Cooperative Extension serve all people regardless of race, color, age, sex, religion, disability, or national origin. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, M. Scott Smith, Director, Land Grant Programs, University of Kentucky College of Agriculture, Lexington,and Kentucky State University, Frankfort. Copyright 2011 for materials developed by University of Kentucky Cooperative Extension. This publication may be reproduced in portions or its entirety for educational and nonprofit purposes only. Permitted users shall give credit to the author(s) and include this copyright notice. Publications are also available on the World Wide Web at www.ca.uky.edu. Issued 02-2011

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Comments

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Dennis
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Thank you SO much for your site! I broke an egg open this morning and there was a “meat spot” in it, and I did not know what it was until I found your awesome website.

Linda
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you are my favorite blog page. Thank you for so much information.

Patsy Lucas
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Very informative, thank you! The sloggers would be fabulous to have!! 🎉🐔🐔

Linda Beus
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I learn so much form you. Truly appreciate it!

Jessie
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Thank you Kathy for everything you do and all your invaluable information! You’re amazing!!

Christy Berry
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I am just learning about chickens. There is a lot of information to go through. Thank you so much for sharing your knowledge.

Candis
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Love this blog so much info !

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