Flightless and poorly flighted birds
All birds are descended from birds that could fly. However, over half of New Zealand’s native land birds became flightless, had reduced powers of flight (such as the kōkako and saddleback), or flew reluctantly. Some, such as the moa, kiwi, takahē and kākāpō, are well known. Also noteworthy is that of the world’s four known species of flightless passerine birds (perching birds), three were New Zealand wrens. The other three native wrens can fly short distances.
The little New Zealand wrens are the most primitive living passerines (perching birds). The three flightless species are now extinct. So is the flighted bush wren. Two species survive. The rifleman or tītiti pounamu is New Zealand’s smallest bird, weighing just 7 grams. The rock wren is a little alpine bird that lives under rocks beneath winter snows.
Where flight is an advantage
For birds that live among mammals, flight is an advantage for two reasons. Firstly, some mammals are effective predators, and flight is a convenient way to escape. Secondly, they compete with mammals for food. Some mammals, such as possums, can climb trees, but most eat a range of foods at ground level – whether they are browsers or predators. Flight enables birds to specialise in foods beyond the reach of most mammals, and to stay out of reach themselves.
Disadvantages of flight
Before humans reached New Zealand, the country had no predatory land mammals. The main predators were birds – eagles, falcons, harriers and owls. Flying was therefore risky for other birds. More effective strategies to avoid being caught were camouflaged plumage, feeding at night, seeking cover and staying motionless.
The ability to fly was no great advantage, and had considerable costs. Flying uses more energy than walking, requiring more high-energy food. Also, the extra muscles and skeletal structures needed for flight account for nearly a quarter of a bird’s body weight. If a bird could find all the food it needed close at hand, and flying was not necessary to escape predators, then flight became an expensive luxury.
Introduced mammalian predators
The majority of the world’s flightless birds evolved on mammal-free islands, and their extinction was mainly caused by people and introduced predatory mammals. When rats, stoats and human hunters reached New Zealand, some of the most interesting and unusual birds could not survive.
Diet of flightless birds
Most New Zealand flightless land birds belong to one of two groups, according to how they find food. Birds in both groups could safely eat low-growing vegetation, or prey in soil or leaf litter, without having to fly away.
Kiwi, snipe (tutukiwi, Coenocorypha species), weka and flightless wrens eat ground-dwelling prey – worms and larvae in soil, bugs in leaf litter – that is available year-round. In other countries, flighted birds also take this prey, but they fly to escape predatory mammals.
The birds in the other group of ground-dwelling birds are herbivorous – they eat leaves, which do not provide enough energy for flying. Leaves are also hard to digest, so these birds need a longer and heavier digestive tract, and tend to be larger than those that eat more nutritious foods. They included the 2–3-kilogram kākāpō and takahē, the huge North and South Island geese, and nine species of moa, including the giant moa (Dinornis giganteus) that weighed up to 270 kilograms.
Two tree-dwellers that eat leaves
The tree-dwelling kōkako and kererū also eat leaves. Kōkako also eat some other foods, and have limited powers of flight. Kererū have full flight, and gain their extra energy from the more nutritious parts of the plant such as fruits and buds.
Large browsing birds
In most of the world, it is mainly land mammals that browse and graze. But in New Zealand, where the only land mammals were bats, some birds became ground-dwelling, flightless and plant-eating. Each species within this ecological guild (animal group with related ecological niches) ate a particular range of foods. These browsing birds were the equivalent of kangaroos and possums in Australia, or the giraffes, elephants and other plant-eating mammals of Africa.
Today there is fierce competition for these foods from many introduced mammals including rats, wild pigs, feral goats and deer, and large areas of forest have become farmland.