The Chickens Egg

An egg in a nest laid by a chicken

What came first, the chicken or the egg?

The egg definitely came before the chicken I am afraid to say. Female animals of many different species including birds, reptiles, amphibians, mammals, and fish, have laid eggs long before the chicken. Humans have been eating them for tens of thousands of years.

What makes up a chickens egg?

Bird eggs consist of a protective eggshell, albumen or egg white and vitellus or yolk, contained within various thin membranes and completely enclosed by a coating called the cuticle or bloom.

Below: Chickens eggs come in many sizes and colours.

The most popular choice for egg consumption by humans are chicken eggs although we eat a variety of others as well. My favourite by a country mile is Guinea fowl eggs.

Classification : Due to their protein content, the United States Department of Agriculture (and most authorities around the world) categorise eggs as Meat within the Food Guide Pyramid. Despite the nutritional value of eggs, there are some potential health issues arising from egg quality, storage, and individual allergies. Egg yolks and whole eggs store significant amounts of protein and choline. 

Definition: An egg is the organic vessel containing the zygote in which an animal embryo develops until it can survive on its own; at which point the animal hatches. An egg results from fertilisation of an ovum. Most arthropods, vertebrates, and molluscs lay eggs, although some, such as scorpions and most mammals, do not.

Quantity: Egg laying animals are kept throughout the world  and mass production of chicken eggs is a global industry. In 2009 approximately 62.1 million metric tons of eggs were produced worldwide from a total laying flock of approximately 6.4 billion hens. 

Size: Egg size tends to be proportional to the size of the adult bird from the half gram egg of the bee hummingbird to the 1.5 kg egg of the ostrich. Kiwis have disproportionately large eggs, up to 20% of the female's body weight. This evolutionary trait results in kiwi chicks that hatch far more developed and able to regulate their body temperature. They emerge from the nest days after hatching, ready to start foraging for food.

There are many current debates about methods of mass production which is why the home husbandry of a few hens is becoming so popular. 

Eggs laid on land or in nests are usually kept within a warm and favourable temperature range while the embryo grows. When the embryo has  developed it hatches by breaking through the shell. Most chicks have a temporary egg tooth on the top tip of the beak that they use to crack or pip the shell and then break free completely.

Bird eggs are laid by females and incubated for a time that varies according to the species and a single young hatches from each egg. There have been occurrences of twins in chickens, I myself have incubated a double yolker on two occasions but they would never survive in the wild and selection pressure will be against occurrences of twins in eggs. Average clutch sizes range from one (condors) to as many as 17 (partridge and guinea fowl). 

Birds lay eggs even when not fertilised it is not uncommon for pet owners to find one their chickens nesting on a clutch of unfertilised eggs.

In 2012, the European Union banned battery husbandry of chickens.

What is a chickens egg?

The anatomy of an egg:

Shell - Made almost entirely of calcium carbonate (CaCO3) crystals in a protein matrix.. It is a semipermeable membrane allowing oxygen to pass into and Carbon dioxide and moisture can pass out through its pores. Tiny pores in a bird eggshell allow the embryo to breathe and the domestic hen's egg has around 7500 pores. The shell also has a thin outermost coating called the bloom or cuticle that helps keep out pathogens.

Membranes – There are two, the inner and outer membranes lying between the eggshell and egg white. Both are transparent protein membranes providing defence against bacterial invasion. Made partly of keratin, a protein found in hair,  they are surprisingly strong and need quite some force to break. You can see this when you bounce a shell less egg like a ball.

Air cell - The air space forms between the inner and outer membranes when the contents cool and contract after the egg is laid. The air cell usually rests between the outer and inner membranes at the egg’s larger end. It can be in a different part of the egg or damaged, in which case you will see moving around when you candle the egg. It accounts for the crater seen at the end of a hard boiled egg. The air cell grows larger as an egg ages and it loses moisture.

Albumen - The egg white, known as the albumen, which comes from albam, the Latin word for white. Four alternating layers of thick and thin albumen contain approximately 40 different proteins, the main components of the egg white in addition to water.

Chalazae - Opaque ropes of egg white looking like a twisted cord, the chalazae hold the yolk in the centre of the egg. They anchor the yolk to the membrane lining the eggshell. The more prominent they are, the fresher the egg.

Vitelline membrane -     The clear membrane that encloses the yolk.

Yolk - The yolk is and emulsified mixture of protein, water and fat. It contains nearly all the vitamins and minerals including iron, vitamin A, vitamin D, phosphorus, calcium, thiamine, and riboflavin.

The yolk is also a source of lecithin, an effective emulsifier. Yolk colour ranges from a pale insipid almost white with a hint of yellow to a magnificent deep orange.  What you feed the bird ,the breed of the hen and how it is kept will all influence the colour of the yolk.

Below: The stages of yolk development in chickens:

Chickens have as many as a dozen yolks at a time getting ready to be laid as you can see from the picture above.

Yolk colour can be changed with diet.

Shape - Bird eggs have an oval shape with one end rounded and the other pointed. This shape varies from species to species. A trait that has arisen by evolution and natural selection and has biological significance.

A pointed egg will tend to sit on its side, with the big end tipped upward. The big end contains the air sac and its shell has a higher density of pores than the more pointed end. Tipping the big end upwards improves oxygen flow to the large head, with the physiologically demanding eyes and brain, that develops in the big end while the tail develops at the more pointed end.

Cliff-nesting birds often have conical eggs as they roll around in a tight circle. In contrast many hole nesting birds have more spherical eggs.

It is the calcification in the shell gland or uterus that fixes the shape but the egg is that shape beforehand.

Below: You can see from this soft shelled egg that it is already egg shaped.

Eggs are shaped before the shell is laid down. 

Below: The egg is laid with the blunt end appearing first.

An often-repeated but incorrect theory of egg shape formation suggested that the oval shape of eggs was caused by the egg being forced through the oviduct by peristalsis, the contraction and relaxation of the muscles which push the egg down the oviduct cause the spherical egg membrane to distort slightly into an ovoid shape with the blunt end caudal or furthest down the oviduct and closest to the cloaca. 

With X-ray photography studies of egg shell formation in bird species we know that egg shape is determined in the oviduct isthmus, before shell calcification. 

These observations are not explained by peristalsis, the egg acquires its shape with the pointed egg caudal as it is forced through the narrow isthmus. 

Muscles contract the oviduct behind the egg pushing it forward in much the same way as the intestines work. The process is called peristalsis.

The shape of eggs varies across bird species, ranging from near-spherical like little bee-eater to pyriform or conical ones such as those of the common murre. The familiar shape of the chickens lying between.

Below: A diagram showing all the parts of an egg.

Egg development and laying process.

A female chick is born with thousands of tiny ova or undeveloped yolks in their ovaries. Once she reaches maturity and begins to produce eggs she releases  an ovum into a canal called the oviduct and begin its journey of development.

From the time an ovum leaves the ovary, it takes approximately 25 hours for the egg to reach the vent for laying. During that time, the yolk will grow larger  and is then surrounded by albumen (egg white), wrapped in a membrane, and encased in a shell.

Below is a x-ray photograph of an egg being laid by a chicken.

Pigment deposits into (Blue / Green egg layers) or onto ( brown egg layers) the shell as the last step of the egg production process. You can scrub the brown colour off Welsummer eggs but not the green off Javanese bantams.

At any given time a productive hen will have eggs of several stages within her reproductive system. The eggs most recently discharged from the ovary are tiny yolks not much bigger than a pinhead, and the eggs farther down the oviduct are larger and more developed.

If sperm is present fertilisation will take place before the albumen forms. Every egg yolk has a white disc called a blastoderm. It is visible to the naked eye but is pale. In an infertile egg the blastoderm is solid white. In a fertile egg, the disc has a faint or distinct ring that makes it look like a doughnut or bulls-eye.

Hens do not usually lay eggs in the dark but I have found eggs on the dropping boards whilst the hens are roosting so it is not impossible. After dusk she will usually not lay till the following morning.

Eggshell production drains calcium from the hen’s body. The comb, wattles, legs, and ear lobes will fade as the calcium leaches out. Calcium must replenish through either feed containing calcium, supplements such as oyster shell, or high amounts of calcium in the soil of birds with outdoor access.

Two shell membranes (inner and outer membranes) cover the albumin and then the eggshell forms by deposition.

The chicken eggshell is 95-97% calcium carbonate crystals in a stabilised protein matrix. The protein is chitin the same as hair and fingernails and without it the crystal structure would be too brittle to keep its form.

Below: This is a picture of the surface of an egg magnified between 5X.

This organic protein matrix has a role in deposition of calcium during the mineralisation process. While the bulk of eggshell is calcium carbonate, it is now thought that the protein matrix has an important role to play in eggshell strength. These proteins affect crystallisation, which in turn affects the eggshell structure. Moreover, the concentration of eggshell proteins decreases over the life of the laying hen, as does eggshell strength.

Below: Photo of egg membranes magnified.

The structure and composition of the avian eggshell serves to protect the egg against damage and microbial contamination, prevention of desiccation, regulation of gas and water exchange for the growing embryo and provides calcium for embryogenesis.

Eggshell formation requires gram amounts of calcium being deposited within hours, which must come via the hen’s diet. During the early phase of shell formation a firm bond is established between the shell membranes and the first crystals to seed from the supersaturated solution of calcium carbonate in which the egg is bathed.

In the shell gland (like a mammalian uterus), mineralisation starts at the mammillae. The shell gland fluid contains very high levels of calcium and hydrogen carbonate. The thick calcified layer of the eggshell forms in columns from the mammillae structures called the palisade layer.

Below: Photo micro-graph of the pore in the shell of a chickens egg.

Between these palisade columns are narrow pores that traverse the eggshell and allow gaseous exchange. image above shows pore in egg shell magnified x 50.

The calcification of the eggshell is the result of a precipitation phenomenon occurring on the eggshell membrane. It happens during passage of the egg through distinct regions of the oviduct and is amongst the most rapid mineralisation processes known in biology with a precise temporal and spatial control of its sequential formation.

Below: Brown chicken egg with irregular calcification. This is extra calcium deposited on the surface.

The proximal (white) isthmus of the oviduct  deposits the fibrous chicken shell membranes and the distal (red) isthmus mammillae or mammillary knobs deposit on the surface of the outer membrane in a regular array pattern. The mammillae are proteoglycan-rich and to control calcification. 

Above: An image of irregular egg shell. This is an irregular surface where the shell has not been deposited properly or evenly. This may be caused by several things including water shortages, stress and overcrowding and illness. Younger and older hens are more prone.

Hens that have been frightened and put off laying may start again with odd shaped eggs or extraneous calcification of the shell. Rough shells on their own are seldom a cause for concern in the main except if you sell your eggs, they can look a little unsightly.

If it persists or gets worse it may be a sign of infection and investigated.

The cuticle forms the final, outer layer of the eggshell.

See 37 egg and shell problems with chicken eggs.  

How long does it take to produce an egg?

In an average laying hen, the process of egg formation takes around 26 hours.

Pigmentation is added to the shell by papillae lining the oviduct, colouring it any of a variety of colours and patterns depending on species. Since eggs are usually laid blunt end first, that end is subjected to most pressure during its passage and so shows the most colour.

Although a hen has only one exterior opening (the cloaca or vent) for egg laying and elimination, eggs are not contaminated during the laying process. Two separate channels, the oviduct and the large intestine, open into the cloaca. As the egg nears the end of the oviduct, the intestinal opening blocks. The egg passes through the cloaca without contact with waste matter.

The typical interval between eggs laid is about 25 hours, so a hen that lays an egg every day will lay a bit later each day.

Egg teeth - Hatching birds, amphibian and egg-laying reptiles have an egg-tooth used to start an exit hole in the hard eggshell. 

The importance of eggshell quality.

In the natural environment eggshells must be strong enough to prevent cracking in order to preserve the embryo until hatching. On the farm shells need to be strong enough prevent damage from handling and to preserve eggs during transport to market.

Eggs are a naturally packaged food and that needs to be preserved to keep the value of the investment and to maintain product quality as long as possible. the Shell is the barrier against infection and should be free from all defects. To this end egg produced commercially have very bright lights shone through them and are tapped. the cracked eggs make a odd noise and can be removed from the production line.

Egg variations.

Young pullets often lay small, shell-less or malformed eggs before getting established in a normal laying routine. Older hens may lay abnormal eggs due to age, stress, or illness.

Pullet eggs, the first ones produced by each young hen, are smaller than the eggs that the same hen will produce as an older hen.

Wind, fart, dwarf, or oops eggs are terms for smaller ones, often with no yolk but that quickly pass through the oviduct without reaching full size. Sometimes they contain only a yolk. In the old days people used to think that rooster laid yolk-less or wind eggs.

Shell-less eggs are released before they have time to develop a shell. They may have membrane holding them together or be loose yolk and white.

Double eggs or an egg inside an egg happen when an egg with a shell is encased by the next egg in the oviduct and a shell is produced over the outer egg as well. I had several of these over the years and they seem to be most associated with storms disturbing roosting hens. The outer one is white only with no yolk.

Double yolkers may have a normal amount of egg white with two or more yolks. it is possible to have three, four or more yolks but this is rare.

An egg will come out with a wrinkly, misshapen, rough, bumpy, or coloured shell.

Egg size is dependent on breed, age, and weight of the hen. Larger chicken breeds tend to lay larger eggs; bantam breeds lay small eggs. Older hens tend to lay larger eggs than younger hens.

The shell colour is a breed characteristic. Most chicken breeds lay light-to-medium brown eggs. A few breeds lay white, dark brown, green, blue, or cream coloured eggs.

Shell colour is only skin deep, inside they are the same. Below: A selection of egg from my chickens.

The shell colour intensity of eggs laid by one hen can vary from time to time, with an occasional darker or lighter shades. 

While most eggs have a slight sheen to the shell, some breeds or individual hens tend to lay eggs with a chalkier texture.

Chicken-and-egg behaviour:

Most hens will lay eggs in the same nest box as flock mates sometimes even cramming two or three birds in a box like sardines in a can. It is not necessary to have a nest box for each hen. One box to three or four hens is the accepted ratio.

Some hens like to lay their eggs in private and others, like these two above will join their sisters in the nest box. 

Sometimes a hen will sit on previously laid eggs and add her egg to the clutch. Another might prefer to sit in another area and deposit one egg by itself.

Often a hen will sing her egg song before, during or after she lays an egg. It is a cheerful quite loud clucking that seems to be a proud announcement. It is the noisiest a hen will get and some do not do it.

Chickens learn by example, so a fake or real egg left in a designated nest box may encourage hens to lay there instead of on the floor or outdoors.

Unconfined hens may lay eggs anywhere outdoors if they don’t want to return to the nest box. Sometimes a free-ranging hen will go missing and reappear weeks later with a parade of chicks. This is one of the delights of being a hen keeper, playing hunt the nest and trying to outwit your little darlings.

Chickens like to eat eggs, even their own. An egg that gets accidentally broken will be eaten by one of the chickens. If you find pieces of shell or egg yolk in the nest box on odd occasions it is usually nothing to be concerned about.

Some chickens become habitual egg-eaters that break eggs open and eat them. Treat egg eaters or culled them from the flock if you wish to have eggs for the kitchen. Not only will that chicken continue to eat eggs, but others will learn from watching and you may end up with several egg-eaters.

Holes in eggs and cracked eggs does not necessarily mean there is an egg-eater in the flock. A hen can accidentally crack an egg in the nest when she sits down or adjusts the nest to lay her own egg. Boredom leads a chicken to peck at stuff and that can be an egg even without the intention of eating it. Make sure it is the chickens, crows and magpies will eat eggs.

Chickens can be fed their own eggs cooked and never raw. Eggs provide protein and the calcium in the shell is beneficial for laying hens.  Use a grater or potato masher to break boiled eggs into pieces of egg and shell.

You can feed used eggshells from the kitchen back to chickens as a calcium supplement without developing egg-eaters. To be safe cook and crush the shell