The Bird's Egg

The egg cell of a bird is really what we call the yolk. In the chicken's egg this yellow ball of yolk is about 20 mm in diameter. The yolk is the food supply for the developing embryo. The nucleus of the bird's egg can be found in the middle of a white spot which is found on top of the yolk. You need a microscope to see the nucleus itself but the white spot is visible to the naked eye.

Johnny's Fact File No.27 The largest egg was discovered in Southern Madagascar. It belonged to the now extinct elephant bird. This egg can be seen in the Academie des Sciences in Paris. It is 326 mm by 390 mm and, with its contents, is thought to have had a mass of 12,25 kg. The smallest egg is laid by the vervian humming bird which is found in Jamaica. The egg is less than 10 mm in length and has a mass of 0,386g. The largest egg of a living bird is that of the ostrich. The ostrich egg is 10 cm to 15 cm in diameter and has a mass of up to 1,78 kg. It takes 40 minutes to boil it!

The egg that a hen (a female bird) lays is covered in layers of albumen (a material like jelly), two membranes and a hard shell. The shell and the membranes protect and support the egg. The albumen feeds the developing embryo with water and protein. The albumen, membranes and shell are added to the yolk layer-by-layer as it passes down a tube called the oviduct inside the hen's body.

The oviduct carries the egg from the ovary out of the hen's body. The last layer to be added is the shell. If a bird's egg is to be fertilized, mating must take place so that the egg will meet a sperm cell in the oviduct before these layers are added.

The shell of a bird's egg is often coloured or patterned to help camouflage it from predators. This is especially important for birds which nest on the ground, such as plovers. A hen bird may lay from 1 to 19 eggs in her nest. A group of eggs laid by a hen is called a clutch.

It does not matter whether the hen bird lays 2 or 20 eggs in her clutch, she will only lay one egg per day. This is important because birds' eggs are very large. If the hen bird were to lay all her eggs at once she would have to produce smaller eggs or fewer eggs.

The hen bird usually lays her egg for the day early in the morning. This is so that she can be free to hunt for food during the morning and in the middle of the day.

Taking Care of the Eggs
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Birds take great care of their eggs. Usually the eggs are laid in a nest. Sometimes the nest can be a simple scrape in the ground or it can be a very elaborate nest. For example the mallee fowl of eastern Australia builds a huge nest of decomposing vegetation which helps to keep the eggs warm.

In most cases it is the body heat of the parent bird which keeps the eggs warm. Keeping the eggs warm is called incubation or brooding. If the eggs get cold the embryo developing inside will die. In some types of birds the hen develops a patch on her breast without feathers. The eggs that she is incubating will be in contact with the skin and so they will be close to her body temperature. The hen must go and feed from time to time, so the cock (the male bird) may take over.

The Emperor Penguin

This is particularly important for one species of bird, the emperor penguin. This is the largest species of penguin, it has a mass of 16 kilogrammes and it lives on the continent of Antarctica. Antarctica is mostly covered in solid ice but it is there that the emperor penguin breeds. It is the male bird that looks after the egg to begin with. The hen lays only one large egg per year. She uses up a lot of her food reserves to make this egg, so she must go to sea to fish for food to replace these reserves. The male stays on the ice and keeps the egg on his feet covered by a fold of skin. Obviously the emperor penguins cannot build a nest from the ice, so the male keeps the egg on his feet for eight weeks through the fierce Antarctic winter. The temperatures can drop to - 60° C.

The chick hatches at about the time when the hen returns with food. At last the cock can go and hunt for food. He will return about two weeks later with more food for the chick.

Shortcut to find out what happens inside the egg.

• Introduction to Animal Reproduction
• Mating on Land
• Reproduction in Mammals
• Different Ways of Growing
• Chapter Summary (useful for revison)
• Questions relating to this chapter
• Top of Page

© Paul Billiet and Shirley Burchill 2008