December 30, 2022
Eggs are one of the most versatile foods around. They can be the star of the show (omelette, quiche), the supporting actor to other ingredients (pad thai, spaghetti carbonara) or the hidden hero by making a cake rise or giving a soufflé its fluff.
Though they are ubiquitous in the culinary world, the process of how an egg forms inside a hen is not widely understood.
An egg develops through multiple stages during its time inside a hen. A yolk is encased in layers of egg white and protective membranes, sealed within a shell and coated with a fast-drying protective fluid.
How Long Do Hens Lay Eggs?
The hen breeds used in commercial egg farming start laying eggs at around 16-18 weeks (four months) old.
The first eggs laid by hens at this young age are relatively small at about 45-50 grams. But over a period of about three months, the egg size increases to an average of 60 grams.
Commercial laying hens continue to produce first quality eggs until they are about one and a half years old. After this, their eggs become fewer and shell quality decreases which leads to more cracking and breaking.
Do Chickens Need a Rooster to Lay Eggs?
It’s a common misconception that a hen is incapable of laying eggs without a rooster around.
There are no roosters on commercial egg farms and hens lay only unfertilised eggs.
A hen requires around 24-26 hours to produce an egg and after it is laid, the process starts all over again about 30 minutes later.
The only reason to have roosters included among a flock of hens would be to produce fertilised eggs as part of breeding programs. This process happens in specialist hatcheries, not on egg farms.
How Is an Egg Formed?
A laying hen’s ovary holds thousands of tiny ova, or future egg yolks. Birds are unique among animals because only one ovary (the left) matures to the stage where it releases eggs.
When a yolk is ready, it moves out of the ovary and into the oviduct - a tube-like structure that is divided into different sections. Over four hours, the yolk moves through an area of the oviduct called the magnum where egg white protein is added to it.
There are many different proteins that make up the egg white. The different protein layers provide protection for the yolk and create a template for the formation of the shell membrane and shell.
After quickly moving through a section called the isthmus where shell membrane fibres are produced, the egg enters the shell gland where the shell forms over about 20 hours. The process is called calcification as layers of calcium carbonate are added to form the shell.
During the last two hours of shell formation the bulk of the pigment (white or brown) is produced and deposited into the outer layers of the shell. This includes the cuticle which is laid down to provide protection against bacteria that might try to penetrate the shell and potential loss of water from the egg’s contents.
The egg then rotates just before laying to be laid large end first, having moved through the oviduct small end first.
Do Chickens Lay Eggs All Year Round?
Adult hens need about 14 hours of daylight to lay eggs consistently and different lighting programs are used depending on the farming system, geographic location and season. The different farming systems include outdoor enclosures that use natural daylight and enclosed sheds that predominantly use artificial lighting programs.
Most commercial farms use artificial lighting to some degree to get a consistent level of egg-laying all year round.