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How Are Chickens Eggs Fertilized?

Maybe you want to raise some fluffy little chicks for your flock or to sell. Perhaps you’ve heard that fertilized eggs make healthier eating. Either way, you’ve got a rooster, and you’re not afraid to use him. But now you’re wondering how it is that chickens’ eggs are fertilized.

After all, you don’t need a rooster to get eggs. What does a rooster do, and what does mating look like? Chickens don’t have the same anatomy we do, but they nevertheless manage to get the deed done. Let’s take a look at how are chickens eggs fertilized.

For Chickens Eggs To Be Fertilized, You Need A Rooster

Chicken hens lay eggs regardless of whether a rooster is present. Egg-laying in these birds is triggered by light, specifically by a minimum of 10 hours of daylight. This trigger is supposed to ensure that chicks won’t arrive in the world in winter when there isn’t enough food, and works for this purpose, provided humans don’t supply artificial light.

Hens will keep laying eggs to have a clutch of about a dozen to brood, although humans have bred some breeds not to be broody. This process continues even without a rooster, which is helpful for commercial egg-producing operations that don’t need to keep expensive roosters and backyard poultry keepers who are generally not legally allowed to keep roosters because of the noise factor.

But to get fertilized eggs, you need a rooster. Hens produce eggs; roosters produce sperm. When chickens mate, the sperm fertilizes the hen’s eggs, and fertile eggs can develop into chicks if incubated at constant warm temperatures for a sufficient time.

chicken on eggs

A Brief Guide To Female Chickens’ Reproductive Anatomy

The reproductive system of a female chicken (a hen) consists of the ovary and the oviduct. In most chickens, only the left ovary and oviduct develop, and the right-hand system atrophies. The ovary is midway along the hen’s back, near the left kidney.

The ovary is where the chicken’s ova (yolks) develop. A hen is born with all the yolks it will ever have, and not all of them will grow.

 When an ovum (yolk) has matured, the infundibulum or upper part of the oviduct envelops it and begins its journey downward. This release is known as ovulation. If a rooster’s sperm fertilizes an egg, it will happen in the infundibulum.

The ovum travels down the oviduct to the magnum, where it is covered in albumen (egg white), and then passes to the isthmus, where the egg membranes are laid down. Next, it moves into the shell gland, where the shell is deposited. After 25 to 26 hours, the egg exits the vagina into the cloaca, and the hen lays the egg.

The cloaca is the chamber into which the vagina and the excretory opening let out. When laying an egg, the hen everts the cloaca so that the egg she’s laying does not come in contact with feces.

Some 30 to 75 minutes after laying, ovulation occurs once more, unless laying occurs late in the day, in which case, there will be a day’s interval between lays.

An Even Briefer Guide To Male Chickens’ Reproductive Anatomy

The reproductive system of a male chicken (rooster) consists of two testes and vasa deferentia (sperm ducts). The testes are near the kidneys, midway along the rooster’s back, and here the sperm develop.

As sperm develop during the rooster’s lifetime, they travel down the vas deferens (singular of vasa deferentia) and are stored here. When chickens mate, the sperm is expressed out of the papillae (singular papilla), little bumps at the end of the vasa deferentia, into the male’s cloaca. We will learn how they get from there into the female’s cloaca.

How Are Chickens Eggs Fertilized? Chicken Courtship Rituals

Roosters engage in courtship rituals to attract hens, including wing dropping and tidbitting. When wing dropping, roosters dance around in circles, lowering the wing on the inside of the circle, presumably to look impressive to the hens. They also attempt to position themselves behind the hens.

Tidbitting involves the roosters calling (if they are alphas), picking up choice morsels of food, dropping them on the ground, and repeating the process in a way that shows off their combs and wattles. Beta roosters will exhibit this behavior silently. Either way, they are trying to get into the hens’ good books.

How Chickens Mate

Once courtship has happened (if it does happen), the rooster hops onto the hen’s back. A hen ready to mate will squat with a lowered head and spread her wings. If a hen doesn’t wish to mate, she will run away, often resulting in the rooster chasing her, holding her down, and mating anyway.

The rooster treads on the hen’s back to help him balance and usually grabs her comb to steady himself. Although the ideal ratio of roosters to hens is 1:10, he may have specific favorites he keeps returning to, and they will end up suffering feather loss and even broken skin and bleeding. To prevent this, furnish favorites with hen saddles.

Once the rooster is aboard, he drops his tail, the hen lifts hers and everts her cloaca, and with some chicken gymnastics, their cloacae (plural of cloaca) meet in a so-called cloacal kiss. The rooster’s sperm is expressed out of the papillae, into his cloaca, enters her cloaca, and travels up the oviduct. He releases between 100 million and 5 billion sperm at a time (fewer later in the day).

Chickens mate more in the early morning and evening, and a rooster may mate between 10 and 30 times a day.

The sperm fertilizes that day’s eggs; some remain in sperm pockets along the oviduct and are used to fertilize eggs for the next 4 or 5 days. Should a hen not want a rooster’s sperm to fertilize her eggs, she can eject most of it after mating.

Once the quick mating process is over, the rooster hops off and struts away, and the hen carries on with whatever she was doing.

The Development Of Fertilized Chicken Eggs

Every hen’s ova have a tiny, white spot called the blastodisc on the yolk’s surface, which contains a single female cell. If there is any sperm in the infundibulum when ovulation occurs, a single sperm cell penetrates the blastodisc, which results in its fertilization. The blastodisc then becomes a blastoderm, which begins dividing into 2, then 4, then 8 cells.

These first stages of embryonic development continue until the hen lays the egg. They then subside until the egg is incubated, either by a brooding hen or in an incubator. If the fertilized eggs are maintained with minimal disturbance at 100 degrees Fahrenheit (38 Celsius) for 20 to 21 days, the embryo inside will develop into a chick and hatch.

If the fertilized eggs are not incubated, they will not develop. Nor will they develop in a refrigerator.

To check whether eggs are fertilized, you can “candle” them (hold them up to a bright light in a dark room). After about 5 days of development, it will be difficult to see through the egg, and you will see the developing embryo as a spider’s web of blood vessels.

However, the most reliable (though destructive) way to check whether eggs have been fertilized is to crack one open and see whether the white dot of the blastodisc has become a target-shaped blastoderm. If you have introduced a rooster and some eggs are fertilized, you are probably not wasting your time incubating the others from that batch.

Can You Eat Fertilized Eggs?

You can eat fertilized eggs, and they don’t taste any different (nor is their nutritional content any higher – that is a myth).

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