There are, or were, kiwi, kakapo, takahe, penguin, weka, moa, three flightless wrens and two adzebills, to name but some.
Why should that be?
No Mammal Predators
One important reason is that, before humans arrived about 1000 years ago, New Zealand had no land mammals that preyed on birds. The only land mammals native to these islands were three tiny bat species
With no keenly scented predators stalking them, kiwi and the other flightless birds could safely forage from the forest floor, living and nesting on the ground. Their biggest threat came from other birds - winged predators such as the laughing owl, eagle, goshawk and falcon.
The second reason kiwi gave away their wings is linked. With the ground being so safe, they could save energy by walking. Kiwi re-directed the energy other species invest in wings and flight muscles into the long beak and strong legs it needed to fill its niche on the forest floor.
||While the bones of most birds are light and hollow to help them fly, kiwi bones are heavy and filled with marrow – great for walking and running.|
All that remains of the kiwi's wing is a small vestigial stump, with a tiny cat-like claw on the end. This small stump gave rise to the species’ name – Apteryx - meaning ‘wingless’.
Maori have a more poetic story about why kiwi can’t fly.
Of course, since people arrived in kiwi land, the forest floor has become a place of threat and peril for all New Zealand’s flightless birds.