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Reproduction in Birds : Inside the Egg

Dissected egg showing the yolk, egg cell and strands of white  © Paul Billiet

If the egg does not meet a sperm cell in the oviduct it will still be laid by the hen. But the embryo will not develop because the egg has not been fertilized. The white spot on the yolk will remain white. If, however, the egg has been fertilized this white spot will soon become red with blood as the embryo starts to develop.

A three day old chick embryo. © Paul Billiet

The embryo needs food, water and oxygen to grow, so thin red blood vessels spread out from it over the yolk and then round the inside of the shell. The yolk provides most of the food, the albumen provides more protein and water and the oxygen passes through the shell and the membranes. For a chicken, it takes 21 days from the time the egg is laid to the time the chick hatches.

During the last two days the chick inside the egg begins to breathe with its lungs for the first time. To do this it needs an air supply inside the egg shell. At the round end of the egg there is an air space. The chick makes a hole in this air space so that it can now breathe. The chick also calls to the hen through the shell. The hen, therefore, knows that the egg will soon hatch.

Facts iconJohnny's Fact File No.28
When birds reproduce usually one cock and one hen, will mate and nest together. In a few species of bird, however, one cock will try to attract as many hens as he can to his mating ground.


These include the peacock, the pheasant and the grouse. In these cases the hen builds the nest alone, she incubates and feeds the chicks on her own too.
There are some species of birds where this behaviour is the other way around. In some species of waders the hen will mate with a cock, lay her eggs in his nest and then leave him to incubate the eggs and feed the chicks. Meanwhile the hen looks for another cock.
Most birds find new-mates each year but there are some birds which pair up for life. If one partner dies they may never mate again. Birds which pair in this way are often long-lived. An example of this can be found amongst the albatrosses. The albatross can live for up to 30 years but it does not begin to breed until it is 15 years old. Swans and geese are also known to pair up for life.

To escape from the egg the chick has a special point on its beak called an egg tooth. This egg tooth breaks a hole in the shell and the chick hatches. The chicks of some birds, such as chickens and pheasants, are very independent and well developed from the moment they hatch. This is because these birds nest on the ground so the chicks must be able to run and hide if a predator approaches.

A Table Showing the Development of a Chick Inside the Egg of a Domestic Hen



20 hours The nervous system begins to develop.
24 hours THE EGG IS LAID.
33 hours The brain starts to form.
40 hours The heart begins to beat.
48 hours A blood supply grows out of the embryo to cover the yolk so that the embryo can feed. Another blood supply grows from the embryo to cover the inside of the shell, so that the embryo can breathe.
5 days The wings and legs start to develop.
7 days The embryo is complete but only 2 cm long.
20 days The chick breaks into the air space at the end of the egg. It starts to breathe air with its lungs for the first time.
The chick calls to its mother from inside the egg.
21 days HATCHING.
The chick uses its egg tooth to make a hole in the shell.
The chick will now be cared for and protected by its parents until it is old enough to care for itself.

How birds take care of their chicks

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