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Starting with the Chicken and Then the Egg: Growth and Development

So what is the answer to the age-old question: Which came first, the chicken or the egg? Well, here, you start with the chicken and end up with an egg. Along the way, you discover the reproductive ins and outs of chickens.

When chickens reach sexual maturity

Young female chickens (pullets) of modern breeds, such as commercial strains of Leghorns, start laying eggs at around 18 to 21 weeks of age and are 8 months old when they reach peak egg production. Old-fashioned or heritage breeds of chickens are late bloomers; they start laying eggs around 6 months of age. After a pullet reaches maturity, three things come together to determine when exactly she will lay her first egg:

  • The number of hours of light she sees in a day

  • Her weight

  • Her body fat percentage

For a hen to lay eggs, a rooster’s presence isn’t necessary. For a hen to lay fertile, hatchable eggs, however, a rooster and his healthy reproductive system are vital necessities.

Starting at about 4 to 5 months of age, young roosters (cockerels) reach sexual maturity, producing sperm and acting like roosters. They can remain fertile for several years, although the quantity and quality of sperm that roosters produce decreases as they age.

During molt, and during the period of decreasing daylight hours in fall and winter, a hen usually takes a break and stops laying eggs. Her reproductive tract shrinks back to the size it was when she was a pullet. The rooster, too, takes a break in the short days of winter, and his fertility decreases for the season, to return in the spring.

Reproduction from a hen’s perspective

A female chick is hatched with a pair of ovaries and oviducts (left and right) and all the eggs she’ll ever lay. After hatching though, only her left ovary and oviduct develops. If something goes wrong with the left ovary and oviduct during her life, she doesn’t have a good backup plan.

When a hen is making eggs, or in lay, her ovary looks like a bunch of bright yellow grapes of various sizes. The egg-making process starts when one of the larger grapes is released from the ovary (ovulation) about 30 minutes after the previous egg is laid, usually in the morning, and almost never after 3 p.m.

[Credit: Illustration by Kathryn Born]
Credit: Illustration by Kathryn Born

The big yellow grape released from the ovary will be the yolk of a new egg. The first part of the oviduct, the infundibulum, looks and acts like a catcher’s mitt to catch the released yolk.

If a rooster’s sperm fertilizes the egg, it happens in the infundibulum. From there, the developing egg travels through the rest of the two-foot-long oviduct. In order, the sections of the oviduct are the magnum, isthmus, shell gland, and vagina, which ends at the cloaca from which the egg is laid. The table shows the timeline and the event occurring at each stop in the route through the hen’s oviduct. The total assembly line takes about 25 to 26 hours.

The Egg Assembly Line
Station Time at Station What Part Is Added
Infundibulum 15 minutes Yolk, sperm (if it's a fertilized model)
Magnum 3 hours Egg white
Isthmus 75 minutes Shell membranes
Shell gland 20 hours Shell (obviously), eggshell pigment (optional)
Vagina Not long (a few seconds) Bloom, also called the cuticle (a waxy protective coating)

The rooster’s role in reproduction

A rooster keeps all his reproductive equipment inside. His pair of bean-shaped testicles is tucked up inside the abdomen, along the backbone, just above the kidneys. Male birds differ from their mammal counterparts in another important way — a rooster’s sperm stays fresh at normal (hot!) chicken body temperature, while male mammals must keep their sperm slightly cooler than body temperature in external testicles.

From each of the rooster’s testicles, a tube called the ductus deferens carries sperm to the cloaca. The rooster doesn’t seem to miss having a functional copulatory organ, and mating is accomplished simply by placing his cloaca next to the hen’s cloaca, and depositing sperm there.

[Credit: Illustration by Kathryn Born]
Credit: Illustration by Kathryn Born

What happens after chickens mate

After mating, the hen stores the sperm in the tiny sperm host glands, located between the vagina and the shell gland of the oviduct. The sperm can live in the sperm host glands for about two weeks after mating.

When an egg is laid, some sperm are squeezed out of the glands and they migrate up the oviduct to fertilize the next egg in the pipeline. This is a good backup plan, because if something happens to the man of the flock, the hens can still lay fertile eggs for a while after he’s gone.

Hens will lay fertile eggs as soon as the second day after a sexually active and fertile rooster is introduced to the flock. It may take him a few days to make the rounds and mate with all the hens, so give him a week before expecting to see a high level of fertility in the eggs.

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