Chicken Science

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How a chicken lays an egg is an amazing process. To start with, each baby female chicken, or pullet, begins life with all the potential egg yolks, or ova, that could develop into eggs within her lifetime. Estimates for how many eggs that represents range from 2,000 to 4,000, or even more. But out of this potential total, few hens lay more than about 100 dozen eggs overall.

A pullet has two ovaries at the time of hatch. But only the left one develops to become fully functional. An explanation for why the right ovary doesn’t develop might be to protect a hen’s body from the stress of laying two eggs at the same time.

All the tiny ova hang together like a bunch of grapes from the hen’s backbone, approximately halfway between her neck and tail. Each is about the size of a pinhead. When the pullet matures into a laying hen, one by one the ova grow to reach the size you find inside an egg.

Each ovum takes about two weeks to develop. Meanwhile it receives nutrients through a surrounding network of blood vessels. The vessels then rupture to release the yolk from the ovary. Sometimes an imperfect rupture leaves a harmless spot of blood on the yolk.

At any given time, then, a healthy hen’s left ovary contains ova at various stages of development. An exception is when the hen is taking a break from laying, such as during a molt. Another exception is an elderly hen that is no longer laying. In such cases, all the ova remain small, like in a pullet.

The hen’s ovary is one of the two major parts of her reproductive system. The other major part is the oviduct. The oviduct is slightly more than 2 feet long and consists of five compartments. Each compartment has a different job in shaping the egg.

At the top of the oviduct, just beneath the ovary, is a 3- to 4-inch long funnel called the infundibulum. An ovum that reaches yolk-size falls into the funnel in a process called ovulation. If the hen has been with a rooster, his sperm joins the yolk here.

About 15 minutes after the yolk falls into the funnel, it enters a 13-inch length of oviduct called the magnum. As it spirals through the magnum, it becomes surrounded in egg white, or albumen. This process takes about 3 hours.

Next the yolk enters a 4-inch long section of the oviduct called the isthmus. In the isthmus the egg acquires two thin layers of protective membranes around the albumen. That takes about an hour and a half.

The egg then enters the 4- to 5-inch long shell gland, or uterus, where it spends the most time — about 20 hours. First fluid plumps the egg until it reaches the approximate shape you would recognize as an egg. Then a hard shell, consisting of calcium carbonate crystals, seals the egg. If the shell is any color other than white, this is where it gets its pigment.

From there the egg enters the 4- to 5-inch long vagina at the bottom of the oviduct. Here a fast-drying protective protein solution, called bloom or cuticle, coats the shell. The bloom seals tiny pores occurring between the calcium crystals that make up the shell.

At this point the egg is leading with the pointy end. Just before the hen lays the egg, it rotates so the blunt end comes out first. That way the egg is less likely to crack when it lands in the nest.

The vagina then pushes the egg into the cloaca, a chamber just inside the hen’s vent. The cloaca is where the reproductive and excretory tracts meet — which means, yes, a chicken lays eggs and poops out of the same opening. But not at the same time. Here’s why:

The vagina grips the egg tightly and turns itself inside out as it pushes the egg through the cloaca and out through the hen’s vent. If you happen to see a hen laying an egg, and she happens to be facing away from you, you might catch a glimpse of the bright red vaginal tissue briefly protruding around the edges of the vent. It withdraws back inside the hen as soon as it releases the egg.

By pressing against the intestinal opening, the everted vaginal tissue shuts the opening while the egg is passing through the cloaca. So the egg — surrounded by protective vaginal tissue — emerges clean. Filth you might find on the shell most likely got there in the nest.

A freshly laid egg feels warm, because a hen’s body temperature is about 106ºF. As the warm egg cools to ambient temperature, an air space develops between the two shell membranes. If you put a fresh egg into a bowl of water, it will sink. The older the egg, the larger its air space becomes. Eventually the air space gets big enough to float the egg — a neat trick for determining how fresh an egg is.

The next ovulation typically occurs within an hour after the previous egg is laid. But every now and then, two ova release within a short time of each other and end up inside the same shell. A double-yolk egg! Double yolkers are typically laid by pullets whose reproductive system is not yet well synchronized. But they may also come from heavy-breed hens, often as an inherited trait.

The whole process of laying an egg takes about 25 hours. So a hen lays about one hour later each day. A healthy hen doesn’t lay eggs in the evening. As her cycle progresses and her laying time approaches 3:00 in the afternoon, she’ll skip one day or more. Then she’ll start a new multiple-day laying cycle.

A group of eggs laid within one cycle is a clutch. A typical backyard hen lays about five eggs per clutch before taking time off. Now that you know the details of how a chicken lays an egg, don’t you agree the hen is entitled to an occasional day off?

Authored by Gail Damerow 

Gail Damerow has written many books about chickens. Those available at Tractor Supply include The Chicken Encyclopedia, The Chicken Health Handbook, Storey’s Guide to Raising Chickens, and more. Visit Gail’s blog at gaildamerow.com.