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In this section, we will cover the basics of the female chicken reproductive tract and cycle to give you a good understanding of how the process works. This is especially important because many of the diseases we treat in backyard poultry are related to something going wrong in the egg production process.
Most pullets will reach puberty around 18 weeks of age and are officially classified as laying hens once they have produced their first egg. Check out our Poultry Life Cycle section for more information on key developmental milestones in your chicken’s life.
The two main parts of the female chicken reproductive tract are the ovary (source of yolk) and the ~ 0.6m long oviduct (source of egg white and egg shell) which can be divided into a further 6 components plus the cloaca and vent that act as the passageway to the outside world.
Chickens are born with two ovaries, but only the left one ends up developing enough to the produce yolks. Female chicks are born with > 10,000 ova (immature yolks) and this is the maximum number they will ever have since they can’t create new ones during their life. When a pullet goes through puberty, the ova will start developing into mature yolks. When a yolk reaches the appropriate size (~2cm diameter), it is released. If there is some damage to the ovary when it is released, you may see a small blood spot on the yolk. If two yolks are released at the same time, you will get a double-yolked egg.
Once the egg is released by the ovary, it gets picked up by the infundibulum where it hangs out for about 15 minutes before moving further down the pipeline. If the infundibulum doesn’t manage to catch the yolk or if it allows fluids and bacteria to travel the wrong direction, this material can get trapped in the abdomen and cause serious inflammation (also called egg yolk peritonitis).
The magnum is responsible for adding the egg white (albumin) around the yolk. It is added in swirling layers, which might sometimes appear as thicker white strings attached to either end of the yolk rather than just being clear fluid (chalazae). This process takes about 3 hours total.
As the developing egg continues moving along, the isthmus then adds the inner shell membrane and the outer shell membrane to help contain the albumin. This process takes about 75 minutes. As the egg cools down after it is laid, an air space forms at the egg’s larger end. This will continue to grow as the egg ages and is why you might see a crater at the end of a hard-boiled.
The shell gland or uterus is where the developing egg spends most of its time – usually about 20 hours total. Calcium carbonate stored in the chicken’s bones is pulled in to create the hard shell, which is why it’s really important that laying hens are given calcium supplements as part of their diet. For breeds that produce coloured eggs, pigment is also added to the shell during this process.
Once in the vagina, a thin cuticle layer is added to further protect the surface and then the vagina pushes the egg through the cloaca and vent into the outside world, large end first.
The cloaca (coming from the Latin word meaning sewer) is a common channel that is also hooked into the reproductive tract and kidneys allowing droppings, eggs, and uric acid to leave the chicken’s body.
The vent refers to the opening from the cloaca to the outside world and is what you will see if you look at the backside of your chicken.
Check out this video from the Virtual Chicken YouTube channel, which has some great videos to help you further understand what goes on where.
On average, most hens will lay about one egg every 24 hours, but this will change with season, age, and annual moults. The term clutch size refers to the number of consecutive days that a hen lays eggs before having a day off. In commercial breeds, this can be up to 30 days (30 eggs) whereas some heritage breeds may only go 4 to 5 days and some broody breeds may only go 8 to 12 days before taking a break.
The primary impact of season change from summer into fall is a reduction in egg production. Chickens are stimulated to lay eggs by day length. Long days and increasing day lengths encourage egg production. As we move from summer into fall, day length declines from about 16.5 hours per day to a low of eight hours per day in June.
High producing breeds can lay over 250 eggs per year with optimal conditions and care, but this will decline as they start to age usually by about 10-15% per year. In commercial production systems, hens are retired at about 18 months of age when they go through their first moult. Most backyard chicken breeds usually produce eggs for about 5 to 6 years.
The annual moult is the 8 to 12 week window every year where birds experience a pause in their egg-laying cycle that allows them to replace their feathers and repair their reproductive system to gear up for the next cycle.
One of the key measures of flock productivity is the egg production rate (also called laying rate), which is the total number of eggs produced in a 24 hours period divided by the total number of birds in a flock. Overall, 80 to 90 percent is considered excellent egg production (100 percent = 1 egg per hen per day), but many other factors like nutrition, temperature, and lighting can affect rates. Production will drop to about 50% in winter and 20% when the flock is going through a moult.
Check out this video from the Virtual Chicken YouTube channel, which has some great videos to help you further understand what goes on where.
Unless there is a rooster present in the flock, the eggs that your chickens lay will be infertile and not able to produce a chick even if the hen sits on them to incubate them.
The average egg weighs between 50g and 70g with egg size generally increasing as birds get older. In supermarkets, a medium egg (6) is 53g, a large egg (7) is 62g, and a jumbo egg (8) is 68g. Eggs can sometimes be larger if they have double yolks or if the hens are on a very high protein diet. Hens that produce very large eggs are at risk of having them get stuck in the shell gland or entrance to the cloaca when they are trying to lay (egg binding), which can turn into a life threatening emergency.