Kiwi put far more time and energy into laying and incubating their eggs than caring for their chicks. In relation to the bird’s body size, the eggs, normally white, are unusually large – about a fifth of the female’s weight, in most species. The little spotted kiwi female, only 1.3 kilograms, lays an egg weighing 300 grams.
The North Island brown kiwi takes up to 30 days to produce its 430-gram egg, which in the last days is so large that the bird cannot feed because of the pressure that the egg exerts on internal organs. The egg is up to 65% yolk by volume, almost twice that of most birds’ eggs. The North Island brown kiwi and little spotted kiwi can lay up to six eggs singly in a season (May to February), but the other species normally lay only one.
The large egg takes an unusually long time to hatch – between 65 and 90 days. This is nearly three times as long as for a weka egg, for example. In the case of the little spotted kiwi and brown kiwi, the male alone incubates the egg. The other species share the task.
Kiwi are monogamous and often mate for their adult lives – between 10 and 30 years. But they are not inseparable, and spend over half the time going their own ways.
Just before it hatches, the chick absorbs the remainder of its yolk, which gives a good start to life. It emerges fully feathered.
The parents leave the chick to fend for itself, and by the end of the first week it starts to look for food. The young bird normally stays in the area where it hatched for two or three years before finding new territory. Territories vary in size from 1.6 hectares (little spotted kiwi) to over 40 hectares (North Island brown kiwi). The birds occupy the same area for life, moving from one den to another and fighting fearlessly to defend their ground.
Because kiwi hatchlings are left at such a young age, they are vulnerable to predators. It is speculated they might have become nocturnal to avoid the Haast’s eagle and Eyles' harrier, now both extinct.
Introduced animals are a more severe threat. The most dangerous are stoats, which feast on the young birds. Kiwi chicks can resist stoat attacks once they weigh about 1 kilogram, but this is not until they are 40 weeks old. Other predators include dogs, feral cats and even wild pigs, which dig up kiwi burrows. Weka are known to eat kiwi eggs.
Only about 5% of wild kiwi chicks survive the first six months. Once there were an estimated 12 million kiwi, but by 2006 there were fewer than 100,000.
Protection and conservation
Kiwi were given official protection in 1896. The Department of Conservation (with the Bank of New Zealand) began a programme of research and protection in 1991. One project is Operation Nest Egg, which began in 1994. Eggs are taken from the wild, incubated artificially and the resulting chicks placed in a predator-free ‘crèche’, such as an offshore island. When they have reached a safe weight they are returned to the area they came from.
Offshore islands such as Kāpiti and Little Barrier remain important in kiwi conservation. There are also mainland sanctuaries, such as at Ōkārito and Haast in Westland, where predators have been controlled. Kiwi are also kept in captivity at 14 kiwi houses around New Zealand.