Birds
North Island Brown Kiwi
Apteryx mantelli · also called brown kiwi, kiwi
The kiwi is New Zealand’s national bird and one of the strangest birds alive — a nocturnal, flightless, burrow-digging forager that has evolved so many mammal-like traits it is sometimes called an “honorary mammal.” It is also in trouble, because the predators it never faced have caught up with it.
Overview
The North Island brown kiwi is the most numerous of the five or six kiwi species, living in native forest, regenerating scrub, exotic plantation, and rough pasture across the North Island. Kiwi are strictly nocturnal, spending the day in a burrow, hollow log, or dense vegetation and emerging after dark to probe the ground for food. Pairs hold a territory year-round and advertise it with loud, shrill calls — the male’s rising whistle and the female’s lower, hoarse cry — from which the bird gets its onomatopoeic Māori name.
A bird built like a mammal
Kiwi break most of the rules of bird biology:
- Smell over sight. Alone among birds, the kiwi has its nostrils at the tip of the bill, and it hunts by sniffing out worms and grubs in the soil and leaf litter, sneezing to clear dirt from its nose.
- Heavy bones. The bones are dense and partly marrow-filled, not air-filled, since there is no flight to lighten for.
- Low body temperature and a slow metabolism, closer to a mammal’s than a bird’s.
- Hair-like feathers, facial bristles resembling whiskers, and vestigial wings.
The egg
Kiwi reproduction is dominated by their giant egg, one of the largest relative to body size of any animal. The huge yolk fuels a long incubation — 70 to 85 days — and the chick hatches fully feathered, open-eyed, and able to walk and, within about a week, feed itself. There is no feeding of the young by the parents. In most brown kiwi it is the male that incubates, sitting for most of each day for over two months.
Threats and conservation
The North Island brown kiwi is Vulnerable and, without intervention, declining. Its ancestors evolved in a land with birds and reptiles as the only predators, so kiwi have essentially no response to introduced mammals: stoats kill the overwhelming majority of chicks, and dogs, cats, and ferrets take chicks and adults. Habitat clearance and vehicle strikes add to the toll.
Intensive conservation works. Predator trapping and fenced sanctuaries allow local populations to grow, and Operation Nest Egg — removing eggs from the wild, hatching them in captivity, and raising the chicks in predator-free areas until they weigh enough to survive an attack — has lifted chick survival from around 5% to over 60% at managed sites. Some kiwi populations are now stable or increasing for the first time in decades.
North Island Brown Kiwi: common questions
Why is the kiwi called an "honorary mammal"?
Because it has converged on a mammal-like way of life. It is nocturnal and forages by smell, with nostrils at the tip of its bill and a large olfactory brain region. It has whisker-like feathers around the face, a low body temperature, dense marrow-filled bones instead of the hollow bones of flying birds, burrows like a badger, and is monogamous with a long life and very low reproductive rate. New Zealand had almost no native land mammals, and the kiwi's ancestors filled part of that empty niche.
How big is a kiwi egg?
Enormous for the bird. A kiwi egg can weigh 430 grams, up to about a fifth or even a quarter of the female's body weight — the equivalent of a human woman giving birth to a six-year-old. The egg is around 65% yolk (most birds' are 35–40%), which gives the chick a large food reserve so it hatches well developed and can feed itself within days. The female carries the egg internally for about a month before laying, barely able to eat in the final days.
Can kiwi fly, and do they have wings?
No, they cannot fly. Kiwi have tiny 2–3 cm wings hidden under their shaggy plumage, each ending in a small claw, and no keel on the breastbone to anchor flight muscles. They are ratites, related to the extinct moa, the emu, ostrich, rhea, and cassowary, and — genetic evidence suggests — most closely to the extinct elephant birds of Madagascar.
Why are kiwi endangered?
Introduced predators. Kiwi evolved with no ground mammals, so they have no defences against stoats, cats, dogs, and ferrets. Stoats kill about 95% of unmanaged kiwi chicks before they reach the weight (around 1 kg) at which they can fend off an attack, and a single roaming dog can wipe out adults across a large area. Where predators are trapped or fenced out, or where eggs are hatched in captivity and chicks "held" until they are big enough ("Operation Nest Egg"), kiwi populations recover.