Kiwi do have wings, but they are tiny leftovers that you would never see on a living bird.1Wikipedia. Kiwi (bird) The wings are only part of the story, because nearly every part of a kiwi’s body has changed to suit life on the ground. The most surprising part is where kiwi came from: their ancestors were probably small birds that flew to New Zealand and only lost flight after they arrived.2Paleornithological Research 2013 (Proceed. 8th Internat. Meeting Society of Avian Paleontology and Evolution). Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves
What a Kiwi’s Wings Actually Look Like
A kiwi’s wing is vestigial, meaning it is a leftover part that no longer does its original job. It sits buried under the bird’s bristly, hair-like feathers, so it can’t be seen from outside.3Wikipedia. Kiwi (bird) The upper wing bone is strongly curved and is the most shrunken bone in the wing.4Journal of Zoology. The wing musculature of the Brown kiwi Apteryx australis mantelli and its bearing on ratite affinities It has lost most of the bumps and ridges that a flying bird’s wing bone uses to anchor muscles, so even on a skeleton it is hard to tell where most wing muscles used to attach.5Journal of Zoology. The wing musculature of the Brown kiwi Apteryx australis mantelli and its bearing on ratite affinities
The stub lies against the body, about the length of a finger joint, and in many kiwi it ends in a small claw. Kiwi don’t flap it for balance or flash it in display the way an ostrich uses its wings. Even the Latin name, Apteryx, means “wingless,” a sign of how well hidden the wings are.
That name feeds a common belief that kiwi have no wings at all. In fact every kiwi grows one on each side, inherited from flying ancestors.
A working bird wing has a long upper arm, a forearm and a hand, all lengthened to carry flight feathers, and the kiwi keeps only a tiny version of that layout. The stub has no long flight feathers, so there would be no surface to push on air.
Why a Kiwi’s Body Can’t Get Off the Ground
Flying birds have a keel, a tall ridge down the middle of the breastbone where the big flight muscles attach. Kiwi have no keel at all.6Wikipedia. Kiwi (bird) Neither do the ratites, the ostrich-and-emu family of birds that includes the emu, rhea and cassowary.7Wikipedia. Kiwi (bird) Without it, a kiwi has nowhere to attach the large chest muscles that power a wingbeat. A smooth, flat breastbone like the kiwi’s gives muscle almost nothing to pull against, so even a bird that tried to flap would have very little power.
Feathers are the second problem. In most birds, tiny hooks on each feather lock together into a smooth, firm surface, so the feathers can push against air or water efficiently, even over long flights.8Save the Kiwi. Feathers like hair Kiwi feathers lack those hooks, so they hang loose and look more like hair.9Journal of Creation. The Kiwi: why can’t it fly? They are warm, shaggy and fluffy, which suits a bird living on a cold, damp forest floor.10Save the Kiwi. Feathers like hair A wing needs a continuous surface to hold air as it pushes down, like a paddle, and a loose feather lets air slip through the gaps. A kiwi’s coat behaves more like a mammal’s fur, trapping warm air against the body instead of catching air to lift the bird.
The third problem is weight. Most adult birds have hollow bones that keep them light enough to fly, but kiwi bones are filled with marrow, like a mammal’s or a young chick’s.11Wikipedia. Kiwi (bird) A kiwi therefore carries more weight for its size than a flying bird of the same size would. On the ground, sturdy bones are an advantage. Strong, thick legs let a kiwi walk, run and dig in soil and rotten logs. The female’s large egg sits low beneath an unusually shaped pelvis, so she can still walk while carrying it. Once the body was built around ground life, parts used only in flight had no job left.
Kiwi Ancestors Probably Could Fly
A kiwi-like fossil bird about 16 to 19 million years old has been found at St Bathans in New Zealand.12Sci.News. Proapteryx micromeros: Kiwi’s Ancestor Could Have Flown from Australia It was much smaller than a modern kiwi and may have been able to fly.13Paleornithological Research 2013 (Proceed. 8th Internat. Meeting Society of Avian Paleontology and Evolution). Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves That suggests kiwi probably did not shrink down from a giant ancestor, an idea scientists had long held.14Paleornithological Research 2013 (Proceed. 8th Internat. Meeting Society of Avian Paleontology and Evolution). Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves A smaller ancestor suggests kiwi lost flight and gained size after settling New Zealand, not the other way around.
The kiwi’s ancestor likely flew to New Zealand, possibly from Australia.15Sci.News. Proapteryx micromeros: Kiwi’s Ancestor Could Have Flown from Australia Birds such as the mallard duck and cattle egret still regularly fly between the two countries.16Sci.News. Proapteryx micromeros: Kiwi’s Ancestor Could Have Flown from Australia
The relatives are surprising too. Kiwi are more closely related to Australian and African ratites than to the moa, New Zealand’s extinct giant birds, which means New Zealand was probably settled twice by different ratite ancestors.17PubMed. Independent origins of New Zealand moas and kiwis DNA shows the kiwi’s closest relative was the elephant bird of Madagascar, a giant extinct bird.18PubMed. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution These relatives are spread too far apart to have walked there, which suggests their ancestors flew.19PubMed. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution That overturns an old textbook story, in which ratites were flightless from the start and drifted apart as ancient continents split.
The ostrich-and-emu family lost flight separately in several branches, not once.20PubMed. Tinamous and moa flock together: mitochondrial genome sequence analysis reveals independent losses of flight among ratites Changes in the switches that turn genes on and off during development, rather than lost genes, drove this loss in up to five branches.21Science News Explores. How some birds lost the ability to fly Many of these switches affect how the wing grows in the embryo.22PubMed. Convergent regulatory evolution and loss of flight in paleognathous birds
Why Flying Stopped Being Worth It in New Zealand
Before humans arrived about 1,000 years ago, New Zealand had no land mammals that hunted birds.23Save the Kiwi. Flightless Kiwi and other flightless birds could feed, live and nest on the forest floor safely.24Save the Kiwi. Flightless
Flight is expensive. It uses a lot of energy and demands big muscles that must be fed. If food is on the ground and nothing hunts you there, that cost brings little benefit, and a bird that saves the energy can come out ahead. The savings can go into other things, such as a kiwi’s unusually large egg.
Kiwi were not alone. New Zealand also had the moa, the takahē and the kākāpō, a flightless parrot, which come from very different bird groups. Since they aren’t close relatives, the shared island conditions seem to have pushed each group toward the ground. Islands elsewhere did the same, as on Mauritius, where the dodo lost flight in a similar safe setting.
How Kiwi Find Food Without Flying or Good Eyesight
Kiwi have the smallest eyes for their body size of any bird, and the narrowest field of view.25PubMed Central. Blind free-living kiwi offer a unique window into the ecology and evolution of vertebrate vision Their eye nerve is about a sixth the thickness of an emu’s, so less visual information reaches the brain, and the parts of the brain that handle vision are much reduced.26PubMed Central. Kiwi Forego Vision in the Guidance of Their Nocturnal Activities In a dark forest at night, eyes help little anyway.
Kiwi have the largest smell centres in the brain, for their size, of any bird studied.27PubMed. Anatomical specializations for enhanced olfactory sensitivity in kiwi, Apteryx mantelli Their nostrils sit at the very tip of the long bill, so a kiwi can sniff right where it probes. That lets it find worms and grubs under soil and leaf litter that no eye could see.
The bill tip is also packed with tiny pits full of touch sensors.28PubMed Central. A new prey-detection mechanism for kiwi (Apteryx spp.) suggests convergent evolution between paleognathous and neognathous birds Kiwi may sense the faint vibrations of prey moving underground, though how much they rely on this is still being studied.29PubMed Central. A new prey-detection mechanism for kiwi (Apteryx spp.) suggests convergent evolution between paleognathous and neognathous birds Wading shorebirds such as sandpipers use the same kind of bill-tip organ, which shows that touch-based foraging has evolved more than once.30PubMed Central. The Anatomy of the bill Tip of Kiwi and Associated Somatosensory Regions of the Brain: Comparisons with Shorebirds
Why Flightlessness Puts Kiwi at Risk Today
Stoats were brought to New Zealand to control rabbits but turned to hunting ground-nesting birds.31Animals. New Zealand wants to kill millions of invasive predators to save these fragile birds About 95% of kiwi that hatch where stoats are active die before adulthood.32Animals. New Zealand wants to kill millions of invasive predators to save these fragile birds
A flying bird can take off to escape danger, but a kiwi can only run or hide in a burrow. Chicks are small and leave the nest young, so they are the most exposed. Adult kiwi can kick hard, but dogs and ferrets can still kill them.
New Zealand aims to remove invasive predators such as stoats, rats and possums. Some chicks are raised in safe areas until they are big enough to defend themselves, then released. Protecting nests and trapping predators give chicks a better chance of reaching adulthood.
Visitors can help by keeping dogs on leads in kiwi areas, since even a well-behaved dog can kill a kiwi. Supporting predator-free projects helps fund the work that protects chicks. Sightings in the wild are rare, so the best chances are at sanctuaries or nocturnal kiwi houses.