How a Chicken Makes an Egg and Why Some Eggs are Unusual
As a backyard chicken-keeper, it is not uncommon to find irregular eggs. Do 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, and the vast majority of the time they do not indicate any cause for concern.
Before we get to all the pretty, funky and bizarre egg pictures, it’s important to understand 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. 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, color (if any) and shell. 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.
A hen's reproductive system consists of an ovary and oviduct (a long tube with several parts that have different jobs).1
The following is an actual hen's reproductive tract.1 I have labeled the functions that occur at different junctures along the way. 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 in this photo). The infundibulum is a muscle that essentially engulfs the ovum (yolk) when it is released. The sperm waits in the infundibulum and has a narrow, 15-18 minute window of opportunity in which to fertilze the ova there.
FERTILE AND INFERTILE EGGS
INFERTILE EGGS: Infertile eggs are ones that have not been inseminated by a rooster, and as such, will never hatch chicks. All eggs contain a concentration of cells on the yolk called the blastodisk, which is identified by its light color and irregular shape. When fertilized, the blastodisk becomes known as a blastoderm.
FERTILE 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 blastoderm is also known as the germinal disc. When incubated under particular 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.
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.
Can a double yolked egg hatch? The short answer is: very rarely. While extraordinarily uncommon, miraculously it can happen, watch twins hatch here!
NO YOLK: Also known as rooster eggs, wind eggs, dwarf eggs, rooster eggs or fart eggs (I just report the news, folks, I don't invent it.). Commonly occur with new layers when reproductive system isn’t quite synchronized yet.
Eggs without yolks can occur in older layers when a piece of tissue from the reproductive tract frees itself, fooling the hen’s reproductive glands into treating it like a yolk, creating an egg out of it. The little piece of tissue is visible in this photo:
NO SHELL OR THIN SHELL: I call soft-shelled eggs ‘rubber eggs” as the membrane is soft and pliable. These eggs are commonly produced by new layers, caused by either an immature shell gland or a glitch in the reproductive system when the
shell was not properly added in the shell gland. Can be caused by stress or poor nutrition. To find them occasionally is no cause for concern, to find them regularly can indicate a calcium, phosphorous or vitamin D deficiency.
ODD 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 alarm.
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. These eggs are repaired in the shell gland, resulting in checks/wrinkles. Some of the eggs that follow fall into several of these categories.
ROUGH-SHELLED OR PIMPLED : 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-SIDED: 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.
UNUSUALLY LARGE: 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.
The average 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.
ABOUT SHELL COLORING: All egg shells start out as white eggs. Colored eggs have their pigment added to the shell a little later in the formation process.
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 in 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 AND GREEN 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.
White egg shells have no pigment at all. Uneven, striped, spotted or speckled shell coloring results from the uneven distribution of pigment as the egg passes through the oviduct.
EGG WITHIN AN EGG: This extraordinarily rare 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.
BLOOD SPOT: When a little blood from the ovary joins the yolk down the reproductive tract, a blood spot will be seen in the egg. This usually occurs in older hens that have a genetic predisposition to them or that have a vitamin A deficiency. While it can occur randomly in any egg, less than one percent of all eggs will contain a blood spot.
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 consistently produces irregular eggs, you should consult an avian vet or perform in-depth research based upon your individual circumstances.
1Anatomical 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