How are Chicken Eggs Fertilized?

Not all eggs are fertilized, and a backyard flock can carry on very happily without a rooster at all, laying plenty of nutritious, infertile eggs which are totally incapable of hatching.

If you want chicks, however, you need to start with fertile eggs.

Some Basic Facts About Unfertilized Chicken Eggs


A rooster is required in your flock if you want fertilized eggs. Without a rooster, you will never have a hatch-able, fertile egg.

But you also need to understand that a hen will lay an egg regardless of whether she is sharing her coop with (or has ever even met) a rooster! Unlike most bird species, which lay eggs only once or twice a year after they have mated. Chickens lay at least a few times per week; some will even do so every day (but often not in winter).

It is the natural goal of a hen to lay until she has a clutch of eggs – this is around a dozen, which, if she is broody, she will then sit on in an attempt to hatch them, fertilized or not. When her eggs are taken from her, she will just keep laying. A healthy chicken will only stop laying when she is satisfied with her clutch.

What are Fertile Chicken Eggs?

A fertile chicken egg is capable of developing a chick. You only require fertile eggs if you wish to breed and hatch baby chicks.

How are chicken eggs fertilized? Just like almost all other species on Earth, fertilization requires a hen and a rooster to mate, combining the hen’s egg with the rooster’s sperm to create a chicken embryo within the egg. Mating must occur before the egg is formed within the hen for the egg to be fertile.

A rooster’s sperm can be viable within the hen’s sperm storage tubule (SST) for just over two weeks after mating. A single mating session can produce as many as 14 fertile eggs, depending on how often the hen lays.

A rooster needs to be with a hen for only two days before she is capable of laying a fertile egg, and most will be fertile after seven to ten days. The fertile eggs can thereafter be laid anytime up to three weeks after mating. Lots of chicken keepers will put a hen and rooster together and wait two weeks. They then collect newly laid eggs for hatching.

Note that, just like all other species of men, not all roosters have fertile sperm – occasionally your feathery boy may be infertile, despite his best intentions.

The chick embryo within the newly laid fertile egg will not develop except under the strictest of environmental conditions (see below).

Is there a Number of Hens I should have Per Rooster?

Silky Chickens

If you strive to breed baby chicks, you will need at least one rooster. Roosters without hens can get along fine, but they will fight each other, sometimes to the death, over their right to mate with hens, and roosters will have their own harems which they can be very protective and possessive of. For larger flocks, the more hens you have, the more roosters you will need, especially to maintain healthy genetic diversity. The best mating ratio varies by breed. For example:

  • 12 Leghorn hens to 1 rooster
  • 6 bantam Silkie hens to 1 rooster
  • Generally – 10 hens per rooster

Note that not all areas allow roosters in backyard coops – check your local council regulations.

Poultry mating rituals are quite brutal and there is no such thing as female “consent”. Many roosters will have their favorite hens within a flock. You need to understand that young, virile roosters can be very overenthusiastic and may over-mate and injure hens – so having more hens per rooster is a good idea. You may also elect to separate the rooster from his hens to give them a break.

How can I Determine that an Egg is Fertilized?

An unbroken egg can be checked for fertile status by candling. Hold the egg up to candlelight, a very bright torch, or a special egg candling light to see the embryo and blood spots within. If the egg appears to be opaque or very cloudy, it has been fertilized.

A cracked fertilized egg will have a defined ring around the small white dot in the yolk, like a “bullseye”. The earliest sign of chick development is veining, which are small red lines running along the surface of the egg yolk.

  • Tiny red spots in an egg are broken blood vessels and are unrelated to fertilization.
  • White strings in the egg are called chalaza; this helps suspend and protect the yolk in the shell as well as prevent the egg from breaking. The chalaza dissolves as eggs age – so it will be more notable in farm-fresh eggs.

Is it Safe to Eat Fertilized Eggs?

Yes! As long as you collect eggs from your coop daily, fertile eggs will look and taste the same as normal eggs.

It takes three weeks for a chick to develop and hatch. If you forget to collect eggs which have been fertilized, they will start to show veining development after 3 or 4 days – but only if they are kept in the perfect conditions for incubation. This requires temperatures around 37.8 degrees Celsius and 60% humidity for several hours per day.

As soon as an egg is put in the fridge, chick development stops.


Any egg is great to eat, fertilized or not – as long as it is fresh! Eggs lose nutrients and protein content as they age – so keep them in your fridge and eat them as soon as possible. And remember – farm-fresh eggs are always going to be more nutritious and taste better than mass-produced, supermarket ones which have been stored for an indeterminate period. All the more reason to keep a small flock of backyard hens!