How Do Chickens Lay Eggs?
How Do Chickens Make Eggs? Here's the Step-By-Step Process.
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“I can’t buy your eggs anymore,” was the astonishing announcement made by a college student who had been one of my best customers. I had to know what was going on. “Well, my husband was talking to your husband, and my husband found out that hens poop and lay eggs out of the same opening.” Oh. When some people make up their mind, there’s just no reasoning with them. But we’re reasonable people, you and I, so let’s explore the question “how do chickens lay eggs?” and why it’s not a problem that it comes out of the same opening as you-know-what.
A pullet starts life with two ovaries, but as she matures, the right ovary remains undeveloped and only the left one becomes fully functional. The functioning ovary contains all the undeveloped yolks, or ova, the pullet started out with. Exactly how many that is depends on which egg-spert you ask. Estimates range from 2,000 to 4,000, or even more. At any rate, from the day she enters this world, each female chick carries with her the beginnings of all the eggs she could possibly lay during her lifetime, but few hens lay more than about 1,000 of the possible total.
If you ever have occasion to examine a hen’s innards, you will find a cluster of undeveloped egg yolks along her backbone, approximately halfway between her neck and tail. Depending on the hen’s age and how long she’s been laying, the yolks will range from head-of-a-pin size to nearly the full size you’d find in one of her eggs. In a pullet, or a hen that’s taking a break from laying (such as during a molt), or an elderly hen that’s no longer laying, all of the ova are small because none is developing in preparation for laying the next egg.
When a pullet reaches laying age, or a hen comes back into lay after a break, one by one the yolks mature, so at any given time her body contains yolks at various stages of development. Approximately every 25 hours, one yolk is mature enough to be released into the funnel of the oviduct, a process called ovulation, which usually occurs within an hour after the previous egg was laid.
If ovulation occurs too rapidly, or if one yolk for some reason moves too slowly through the oviduct and is joined by the next yolk, the pullet will lay an egg with two yolks. Double yolkers are typically laid by pullets before their production cycle becomes well synchronized, but may also be laid by heavy-breed hens, often as an inherited trait. Sometimes an egg contains more than two yolks; I once cracked open an egg that had three. The greatest number of yolks on record is nine in one egg.
During a yolk’s journey through the two-foot long oviduct, it is fertilized (if sperm are present), encased in various layers of egg white, wrapped in protective membranes, sealed within a shell, and finally enveloped in a fast-drying fluid coating called the bloom or cuticle.
When the process is complete, the shell gland at the bottom end of the oviduct pushes the egg into the cloaca, a chamber just inside the vent 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.
The shell gland, which technically is the hen’s uterus, grips the egg so tightly that the gland gets turned inside out as it follows the egg through the cloaca and out through the vent. If you come along when a hen is laying an egg, and she happens to be facing away from you, you might catch a glimpse of the tissue — vividly red because it’s loaded with tiny blood vessels — briefly protruding around the edges of the vent before it withdraws back inside the hen as soon as the egg is laid.
This everted, or prolapsed, tissue presses against the intestinal opening to ensure it remains shut while the egg passes through the cloaca. So the egg — having been surrounded by protective uterus tissue — emerges clean. Droppings in a chicken nesting box are the result of activities other than laying, such as lingering in the nest after laying an egg, roosting on the edge of the nest, hiding in the nest to avoid being pecked, scratching in bedding material, and napping in the nest. Any filth you might find on an eggshell got there after the egg was laid.
So now you are armed with an answer to how do chickens lay eggs, ready to allay the fears of any of your friends or customers who might express concerns about the opening an egg comes out of. And by the way, those college students who stopped buying backyard chicken eggs from me did not give up eating eggs. They bought them at the supermarket, where (don’t you know?) eggs are manufactured in sanitary plastic cartons.
When Prolapse Becomes a Problem
Prolapse of the uterus is a natural process by which eggs are laid. If, however, an egg is too large, or a pullet is immature when she begins laying, the uterus may not readily retract back inside. Instead it remains prolapsed, a serious condition in which uterine tissue protrudes outside the vent. Unless you catch it in time, the exposed pink tissue will attract other chickens to pick, and the pullet will eventually die from hemorrhage and shock. Prolapse that progresses to this stage is called pickout or blowout. If you catch it right away, you may be able to reverse the situation by applying a hemorrhoidal cream, such as Preparation H, and isolating the pullet while she heals.
The problem may be largely avoided by preventing your mature hens (particularly heavy breeds) from getting too fat and by ensuring your pullets don’t start laying too young. A pullet that lays before her body is ready is more likely to have prolapse issues.
Under normal circumstances pullets reach maturity during the season of decreasing day length. If you raise pullets out of season, the increasing day length that normally triggers reproduction will speed up their maturity, more so the closer they get to laying age. Maturity may be delayed in pullets hatched from August through March by using controlled lighting.
Consult an almanac to determine how long the sun will be up on days occurring 24 weeks from the date of hatch. Add 6 hours to that day length, and start your pullet chicks under that amount of light (daylight and electric combined). Reduce the total lighting by 15 minutes each week, bringing your pullets to a 14-hour day by the time they start to lay. When they reach 24 weeks of age, add 30 minutes per week for 2 weeks to increase total day length to 15 hours.
Since spring is the natural season for hatching chicken eggs, pullets hatched from April through July and raised in natural light will mature at the normal rate, making them less likely to experience prolapse issues.
Backyard Poultry covers common questions like “how do chickens lay eggs?” in our Poultry section.
Originally published in Backyard Poultry March / April 2010 and regularly vetted for accuracy.