It takes 30 days to form a kiwi egg inside the female.
Its smooth, thin, white or greenish-white shell is about 120 millimetres long and 80 millimetres in diameter.
Because of their size, kiwi could be expected to lay an egg about the size of a hen’s egg. In fact, the kiwi egg is six times as big as normal for a bird its size. They are almost exactly the same size as eggs produced by the now-extinct bush moa, a much bigger bird. This has led to speculation about whether the kiwi was once a very much larger bird.
Kiwi eggs also contain the largest proportion of yolk of any bird – 65%, compared to 35-40% for most birds. Kiwi chicks hatch with a large external yolk sac, which is gradually absorbed through their navel, over their first 10 days of life. This is why kiwi parents do not have to feed their newly hatched chick.
Huge effort to produce a huge egg
To produce such a large egg, the female kiwi must eat three times as much as usual.
The egg grows to take up 15-20% of her body mass and her pregnant belly bulges so much it touches the ground. The female has to walk with her legs wide apart to accommodate it. Sometimes a female will soak her belly in cold water to soothe the inflamed stretched skin and temporarily relieve the weight.
Just before the egg is laid, it takes up so much room inside the female there is little left for food and she will fast for two or three days before laying.
Most clutches are one egg
In most kiwi varieties, the typical clutch size is one egg. The exception is brown kiwi, which usually lay two eggs in a clutch.
Although enormous, the egg is laid quickly and, for brown kiwi and little spotted kiwi females, her work is then done. She leaves the burrow and the male takes over incubating the egg. In the other kiwi varieties, the male and female kiwi share incubation.
In two-egg clutches, the second egg will already have begun to develop inside the female and will be laid about 25 days after the first. It is rare, but not unknown, for a third egg to be produced, especially if one of the first eggs is lost or collected as part of Operation Nest Egg. The most prolific egg producer is the brown kiwi, which will often lay two-to-three clutches each year.
A female kiwi can lay up to 100 eggs in her lifetime.