Eggs Can Be Very Different From Each Other

Egg irregularities are common. Backyard chicken-keepers need not worry unnecessarily about the occasional strange-looking egg. Take a picture of it, discuss it at the water cooler next day and get a good chuckle out of it. They happen fairly often and the vast majority of the time they do not indicate any cause for alarm.  In order to understand why irregular eggs occur, it’s important to understand how a hen’s reproductive system is supposed to work when firing on all cylinders.

A hen's reproductive system consists of an ovary and oviduct 

(a long tube with several parts that have different jobs)

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. 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. An egg requires approximately 25 hours to complete the addition of the egg white, the shell membranes, and the shell. Soon after an egg is laid, the process starts again.

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 essentiallly 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 there.



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.  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


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 will develop into a chick. The blastoderm is characterized by its bullseye appearance of regular, concentric circles.


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.


Triple yolker. Yikes.

Tiny eggs containing no yolk are referred to as rooster eggs, wind eggs, dwarf eggs, rooster eggs or fart eggs (I don't make this stuff up, folks.). 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:

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.



(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.

Rough shelled or Pimpled Eggs

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.

Flat Side

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.

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. 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.


This situation 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.


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.


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.


All egg shells start out as white eggs. Colored eggs have their pigment added to the shell at the end of the shell formation process in the uterus, which is also referred to as the shell gland.

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.


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.

This egg was laid by one of my Olive Eggers.

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.


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