Hannah Lee of Balclutha Primary School asks :-

How do birds develop in an egg?

Caroline Beck, a zoologist at the University of Otago, responded.

Eggs are special cells found in female animals. Nearly all animals produce eggs, and some animals lay their eggs outside their body. Birds lay eggs that have a hard outer covering or shell to protect the baby inside. Inside the shell are the egg white (albumen) and the yellow egg yolk, which are used as food for the developing baby chick.

The eggs that we eat (usually hen's eggs) are not fertilised, and so a baby bird does not grow inside them. If male and female birds are allowed to mate, some of the mother's eggs will be fertilised and a baby bird can develop inside them. First, starting before the egg is even laid, a small part of the yolky bit of the egg divides into smaller parts, or cells, that will make up the new baby. By the time the egg is laid, a few hundred tiny cells make up the beginnings of a baby chick (embryo), which is just a tiny flat white dot on the yolk. To help her baby grow, the mother hen has to sit on the egg to keep it nice and warm.

Some of these cells turn into the backbone of the chick, others will make the heart, yet others will form the brain and eyes. Slowly, over the next few days, the baby chick grows, adding more and more cells. Blood vessels form across the yolk, allowing the food in the egg to be used as energy to make the baby chick grow. After 3 days the baby chick begins to sprout wings and legs, and after 12 days feathers begin to grow. After 3 weeks, the chick has grown so much that it fills the whole shell and has run out of food and air. It first makes a small hole in the shell with its beak and then pecks at the shell until it can hatch into a baby chick.