It is difficult to be certain about the kiwi’s evolutionary history as there are few fossil records – the oldest known fossil is a femur, about 1 million years old, found in coastal deposits near Marton, in the North Island.
Isolated by the changing landscape
New Zealand’s changing landscape and land formation is thought to have influenced the way kiwi evolved. At various times the three main islands of New Zealand (North Island, South Island and Stewart Island) were either joined together, split in different places and into different shapes, or were under water.
As the landscape changed, groups of kiwi became cut off from each other. Because they couldn’t fly, they were kept isolated by physical barriers such as mountains and glaciers, wide rivers and seas, and by harsh terrain, including infertile volcanic soils.
Separated groups could only breed among themselves, sharing a gene pool.
As generations passed, kiwi in each group became increasingly different from kiwi in other groups. Nature selected traits most useful to their local environments and the groups became so different they no longer naturally interbred. Eventually they became separate species altogether.
Changes took millions of years
Researchers think the first species separation happened when the brown kiwi group separated from the spotted kiwi. The next split happened somewhere south of Okarito on the West Coast of the South Island. It is thought that impassable glaciers separated populations of the ancestral tokoeka. During periods of isolation in ice ages, the birds south of the glaciers gradually evolved into the various forms of tokoeka we know today, with Stewart Island birds separating from the rest about 4 million years ago.
It is thought that the group of birds north of Okarito, now known as rowi, extended as far north as southern Hawke’s Bay – rowi remains have been found in pre-European Maori middens there. This was possible because at one point in New Zealand’s history, the sea strait dividing the North and South Islands was further north, running through about where the Manawatu Gorge and Hawke Bay now are.
Brown kiwi ancestors reached the Taranaki area at a time when sea levels were lower and the two main islands were joined by land. When the islands split again, some birds became isolated on the North Island (about 2.5 million years ago) and evolved into today’s brown kiwi species.
At about the same time, the spotted kiwi split into the two species recognised today – great spotted kiwi and little spotted kiwi.
A consistent design
Compared with other bird groups separated for such long periods, the design of kiwi has been remarkably conservative – despite major differences in their genetic make-up there are only slight physical and behavioural differences between species. For example, for a long time taxonomists didn’t recognise rowi as a separate species because their soft brown plumage, calls and shared incubation are more similar to tokoeka than to their closest relative, the brown kiwi. It was only when the rowi’s DNA was studied that it became obvious it is a different species.
Adapting to life in the dark
Kiwi do not see in colour but do have a highly developed sense of smell, a useful adaptation for a bird that feeds at night.
The changes in vision and smell were similar to adaptations found in nocturnal mammals. Only about 3 per cent of bird species are nocturnal, and kiwi are the only nocturnal ratite.