The kiwi’s egg is huge. In proportion to its body size, the female kiwi lays a bigger egg than almost any other bird. In fact, kiwi eggs are six-times as big as normal for a bird of its size.
While an ostrich may lay the world’s largest bird’s egg, it is actually the smallest in proportion to the mother – just 2% of her body weight. By comparison, the kiwi egg takes up about 20% of the mother’s body.
In humans, a baby at full term is 5% of its mother’s body weight.
A female kiwi can lay up to 100 eggs in her lifetime.
While laying such a large egg is painful, there is an advantage. Most bird eggs are 35-40% yolk but the kiwi’s egg is a 65% yolk. The nutritious yolk produces kiwi chicks that hatch fully feathered and independent, and is so enormous that it continues to sustain them for the first week of life. By that time, chicks can provide for themselves and kiwi parents seldom have to feed their offspring.
Why such a big egg?
It is still not clear why the kiwi produces such a large egg. Some researchers believe the kiwi has always been a small bird, and that its egg has grown. Others suggest the kiwi was once much larger and, while the bird shrank over time, its egg did not.
It seems the latter explanation is more likely, because gradual evolutionary changes to an adult bird’s characteristics are more likely to be survived than changes to an egg or foetus.
If the latter theory is true, the kiwi’s ancient ancestor would have probably been about the size of a cassowary, up to 1.5 metres tall.