Story: Large forest birds
Page 1 – New Zealand’s large forest birds
Some strange and outlandish large birds live within New Zealand’s forests. Some of them have evolved in isolation over many millions of years. Others arrived in the country later, and their descendants became bigger than their relatives elsewhere.
A number of large birds of the bush have become extinct within the last few hundred years. But the decline of other species has been halted, thanks to efforts by scientists and the public.
Large native forest birds include the weka and takahē (both rails), kererū (a fruit pigeon), kākāpō and kākā (both parrots), and kōkako. All are endemic species (found only in New Zealand), and are protected by law.
Birds that can’t fly
In the course of their evolution, some of these birds lost one of the most bird-like characteristics – the ability to fly. Of the larger forest birds discussed here, only kererū and kākā are able to fly long distances.
Others have semi-functional wings. Kōkako use theirs like a hang glider for controlled glides. Kākāpō occasionally use their small wings like a parachute, to slow a free fall from low trees and bluffs.
Loss of flight is thought to be an adaptation to certain conditions as these birds were evolving:
- a low risk of predation on the ground, as there were no mammal predators
- a higher risk of predation when flying, from giant eagles, falcons and harriers
- a lack of browsing and ground-foraging mammals, so foods near the ground were abundant
- large, continuous areas of forest, reducing the need to fly from one patch to another.
Some flightless birds have unusual feathers. All five kiwi species and the kākāpō have feathers that are fluffed out for better insulation, not flattened and streamlined for flight. Their metabolic rate (heart rate, body temperature and oxygen consumption) is lower than that of most other birds, which conserves energy. Their large size also reduces heat loss, as they have less surface area relative to their body volume.
Long-lived, slow breeders
Many New Zealand birds are relatively long-lived and have small numbers of offspring. For most of their history there were few predators, so a high rate of reproduction would not have been necessary – but since mammal predators were introduced, this has counted against them.
Free of the need to fly, birds could also become larger. This was an advantage for birds that fed on low-quality leaf material, as this takes time to digest and needs to be carried in the digestive tract long enough to break down and release nutrients. Their size also made it possible to reach higher foliage, and was carried to the extreme in the case of the extinct moa – some were over 2 metres tall.
New Zealand’s birds hold several size records – including the world’s largest parrot (kākāpō) and largest rail (takahē). The parea (Chatham Islands pigeon) is one of the world’s largest pigeons.
The country was also home to an even larger rail (the adzebill), the largest known eagle, and exceptionally large geese – all now extinct. The biggest moa species was among the world’s largest ratites – the group that also includes the ostrich, emu and kiwi.
Foods of the forest
The forests and forest margins have varied resources spread through the various layers – worms, grubs and bugs in soil and leaf litter; grasses and leaves; insects at all levels; and berries in the tops of the highest trees. Among them, these birds have evolved different strategies to exploit this diversity.
When the Polynesian ancestors of Māori arrived in New Zealand around 1250–1300 AD, large and mainly flightless birds roamed the forests and forest fringes. The new settlers developed techniques to trap the birds, catch them with the help of kurī (dogs) or flush them out with fire. But the slow breeding rate of many of these birds could not sustain the level of harvest, and most became scarce or extinct within a few hundred years.
Smaller forest birds met with a similar fate, but for different reasons. They were devastated by the mammal predators that Polynesian and European explorers and settlers brought with them.