Nocturnal Bird

Stewart Island tokoeka is the only kiwi taxa that occasionally steps out in the midday sun.

One reason kiwi are nocturnal is the food they hunt. When the sun goes down, many of the underground invertebrates they like to eat move up closer to the soil’s surface.

It is also thought that kiwi sought the shadow of night to avoid the hunting birds that once ruled New Zealand’s daytime skies, relying on sight and sound to find their prey. The now extinct goshawk, like today’s New Zealand falcon/karearea, was a deadly divebomber.  And the giant Haast eagle with its three-metre wingspan (also extinct), would have snatched and snacked on kiwi, given the chance.

Kiwi eyes

In 2007, a team of researchers released a paper suggesting kiwi eyes have evolved differently to those of most nocturnal animals, which often evolve large eyes to gather what little light there is. While large eyes are heavy, for a non-flyer like the kiwi, weight should not have been a concern.

However, the researchers from the Universities of Birmingham, Lincoln and Auckland, found that kiwi’s eyes are very small and their visual fields are the smallest yet recorded in any bird. The parts of a kiwi’s brain that serve vision were virtually non-existent, making their brains unique among birds.  However, the parts of the brain devoted to touch and smell are large.

The researchers suggest that kiwi evolved differently because the energy they would need to run large eyes would not be balanced by the information gleaned from a dimly lit forest floor, especially since most of their food is hidden underground. Evolution therefore favoured abandoning vision as a key sense, setting kiwi out on an avian evolutionary limb – while most birds get most of their information through their eyes, kiwi sense it through the tip of their beak and their whiskers.