The kiwi is certainly one of the strangest birds in the world.
It is a bird trying to be a mammal. It is flightless, it lives in burrows in the ground, it likes to forage for food on the forest floor and its coat feels more like animal hair than bird feathers.
It is the only bird in the world with external nostrils at the end of its beak - highly unusually for birds, it has a good sense of smell - it is the only bird to have no external tail, and, for goodness sake, it has whiskers like a cat.
There is more. Its body temperature at 38 degC is 2 degrees lower than other birds and to top it all it lays the largest egg, in relation to the size of its body, of any bird in the world. An ostrich egg weighs 2 per cent of the adult female ostrich, a kiwi egg weighs an eye-watering 20 per cent of the adult female.
How did this remarkable bird come to be so strange?
Kiwis are members of a group of birds called ratites. These are defined as having no bone structure to anchor their wing muscles and so even if they had flight wings they still would not be able to fly.
Other members of this group of flightless birds are ostriches in Africa, emus and cassowaries in Australia, rheas in South America and New Zealand's extinct moas.
The kiwi is by far the smallest ratite.
Until recently it was thought that all ratites originated in the huge super-continent of Gondwanaland and that different species evolved from this common ancestor after the split-up of Gondwanaland into pieces such as Australasia and South America. However, examination of ratite DNA shows a more recent split. The current theory is that ancestral kiwis evolved in Australia and probably walked across land bridges or island hopped on debris rafts, thanks to the very different geography millions of years ago.
There are five species of kiwi: the great spotted, the little spotted, the okarito, the North Island brown and the South Island brown. The biggest of these is the 40cm tall and 3.3kg great spotted kiwi, which is found in the mountainous regions of the South Island. The smallest is the little spotted kiwi, 25cm tall and 1.9kg. The little spotted kiwi is extinct on the mainland but 1350 remain on Kapiti Island and there is a programme to introduce them to other predator-free islands.
That still leaves the question of why kiwis try to be like mammals? The answer lies in New Zealand's geological history.
Zealandia is a huge piece of continental crust that split off from Gondwanaland 85 million years ago. Since then Zealandia has been moving north. About 23 million years ago nearly all of it was under water. Today it is estimated that the piece of land we call New Zealand represents only 10 per cent of Zealandia, with the rest submerged.
If, and it is a big if, Zealandia was drowned completely then its flora and fauna came after the split-up from Gondwanaland. Having said that, there is evidence of its flora having Gondwanan origins. Many of New Zealand's birds, bats and plants arrived from Australia, either by flying or being carried in the guts of birds. The dominant large life form on New Zealand was birds, because they could fly.
That is the main reason that bats were the only mammalian life form here. Given the huge time span since the arrival of some of these immigrants and the different climate and geophysical conditions in New Zealand, many have evolved not only into different species but, in some cases, have become so different that it requires detailed DNA analysis to establish that they have an Australian ancestry. The kiwi is such an example.
It could be that primitive mammals came with the Gondwanaland split or even walked across land bridges. However, competition for food or the different, more temperate climate in New Zealand gradually killed them. Whatever the reason, in New Zealand, birds were dominant. The only mammals that survived were three species of bat.
Given no competition from ground-based mammalian life, and that the forest floor contained copious amounts of food, the forest floor was open for exploitation.
That is why some bird species here, such as the kiwi, behave like mammals. Bizarrely, even the bats walk on the forest floor as they hunt for grubs. This unique history makes New Zealand a really special place.
- © Fairfax NZ News
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