In fact, in proportion to its body size, the female kiwi lays a bigger egg than almost any other bird by comparison, an ostrich’s egg is just 2 per cent of its body weight, while a human baby is 5 per cent of its mother’s body weight just before birth.

While such a large egg may be painful for the female kiwi, there is an advantage.  From such a huge egg emerges a mature, fully-feathered and independent chick.  The enormous yolk nourishes it before and after it hatches, with no help needed from the parents.  Most birds’ eggs are 35 – 40 per cent yolk but the kiwi’s egg is a 65 per cent yolk – the most nutritious of all birds’ eggs.  This yolk sustains the chick for its first week of life, by which time the chick can provide for itself.  Kiwi parents seldom feed their offspring.

Why Lay 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.  This is because gradual evolutionary changes to an adult bird’s characteristics (wings or no wings? bigger bird or smaller bird?) are more likely to be survived than changes to an egg or foetus.

If this theory is true, the kiwi's ancient ancestor would have probably been about the size of a cassowary, up to 1.5 metres tall.

In some species, the kiwi male incubates the egg alone for up to 80 days.