Kea: Alpine Parrot That Reasons With Odds

Introduction

In the Murchison Mountains of New Zealand’s South Island, conservation workers kept finding sticks jammed into the wooden boxes that hold their stoat traps. The sticks set off the snap-traps inside, and in some areas, according to a Department of Conservation report cited in a 2018 study in Scientific Reports, up to 70% of the traps were sprung. Over about two and a half years, sticks turned up in at least 227 different trap-boxes up to 27 km apart. When a camera finally caught the culprit, it was a kea: a wild parrot that had invented its own tool use to reach the egg bait.

The kea (Nestor notabilis) is a large olive-green parrot that lives only in the South Island of New Zealand, where it ranges from forest up to the high mountain peaks. A 2021 study in Molecular Ecology describes it as “considered the only alpine parrot in the world”. It is bold, curious and highly social, and its intelligence has made it a favourite subject of animal cognition research. Experiments have shown kea reasoning about probabilities, and field experiments have shown that one of their calls spreads play through a flock.

The same curiosity has repeatedly brought kea into conflict with people. Settlers paid bounties on them for attacking sheep, and around 150,000 were killed. Today kea chew lead from old buildings, scavenge carcasses shot with lead ammunition, and lose adults, chicks and eggs to introduced stoats. The Department of Conservation estimates between 1,000 and 5,000 remain.

Kea at a Glance

Feature	Figure	Source
Length	46 cm	New Zealand Birds Online
Weight	males 900 to 1,100 g, females 700 to 900 g	New Zealand Birds Online
Clutch	usually 4 eggs, range 1 to 5	New Zealand Birds Online
Egg-laying to fledging	about 116 days	Kemp et al. 2018, New Zealand Journal of Ecology
Known longevity	at least 22 years	New Zealand Birds Online
Range	about four million hectares of the South Island	New Zealand Birds Online
Population	1,000 to 5,000 estimated	Department of Conservation
Kea killed under bounty	about 150,000, between 1860 and 1970	New Zealand Birds Online; Dussex et al. 2015
New Zealand status	Threatened, Nationally Endangered	Department of Conservation
Bird of the Year	2017	Department of Conservation

A Parrot of the Mountains

Built for open country

New Zealand Birds Online describes the kea as “a large, strong-flying, olive-green parrot with scarlet underwings and a slender grey-black bill”. The bright underwings show mainly in flight. The species is sexually dimorphic: females weigh about 20% less than males and have a shorter bill. Juveniles have yellow skin around the eyes and at the base of the bill, which fades to grey as they mature.

Its commonest call, according to the same source, is a long, loud, high-pitched descending cry, written as “kee-ee-aa-aa”.

The only alpine parrot

Kea live from coastal dunes to high alpine peaks, according to New Zealand Birds Online, though they are most common in montane forest. They gather on rocky outcrops and windy saddles above and below the treeline. The Department of Conservation places their range across the South Island, from Golden Bay to Fiordland. New Zealand Birds Online notes that they are absent from the North Island and Stewart Island.

That mountain lifestyle has made the kea famous as the only alpine parrot, and a 2021 genomic study by Martini and colleagues set out to test what lies behind it. Comparing whole genomes of the kea and its closest relative, the forest-dwelling kaka (Nestor meridionalis), they found no major functional genomic differences between the two species in pathways associated with high altitude. Signs of selection for traits common in alpine species were present in both, which suggests that alpine adaptation did not drive the split between them. The study’s title points to phenotypic plasticity, behaviour and ecology as the alternative explanations. The authors also found that kea and kaka responded very differently to past climate warming, which highlights potential threats to the kea in a warming world.

A history written in genes

Genetic studies by Dussex and colleagues have traced how kea spread across the South Island. A 2014 study in Molecular Ecology, using 17 microsatellite markers and mitochondrial DNA, identified three genetic clusters and supported a scenario in which kea spread out from a single glacial refuge after the last Ice Age. A 2015 study in PLOS ONE compared contemporary samples from 410 kea with 15 historical and 4 fossil samples. It found a loss of mitochondrial diversity since the end of the last glaciation and a range-wide decline that dated back some 300 to 6,000 years, before Europeans arrived.

A Mind for Probabilities

Reasoning from ratios

In 2020, Amalia Bastos and Alex Taylor reported in Nature Communications that kea show three signatures of statistical inference. They worked with six male kea at Willowbank Wildlife Reserve in New Zealand. Black tokens were rewarding and orange tokens were unrewarding. An experimenter would draw a hidden token from each of two jars, and the kea chose which closed hand to take.

The jars were designed to separate real reasoning about proportions from simple counting. In one test, both jars held 20 black tokens, but one held 100 orange tokens and the other only 4. In another, one jar held 57 black and 63 orange tokens, while the other held 3 black and the same 63 orange. All 6 kea chose the jar with 57 rewarding tokens above chance within their first 20 trials. The authors note that capuchin monkeys fail this control, while kea used the ratio of tokens rather than their absolute number.

The kea then went further. When a barrier was placed halfway down each jar, they judged the odds using only the tokens above the barrier, the ones that could actually be sampled, even though both jars held identical overall mixes of 40 rewarding and 40 unrewarding tokens. In a social test, three of the six birds chose, above chance, an experimenter who had been deliberately picking rewarding tokens over one who had sampled blindly. The authors concluded that true statistical inference exists outside the great apes, and that this kind of domain-general thinking can evolve in a brain structured very differently from a primate’s.

A pebble for a missing beak

Another Willowbank kea, a wild-born male named Bruce, lost the upper part of his bill. A 2021 study in Scientific Reports by Bastos and colleagues found that he preens himself with pebbles, wedging a small stone between his lower mandible and tongue and moving it through his feathers. In over 90% of the instances in which Bruce picked up a pebble, he then used it to preen. In 95% of cases where he dropped one, he retrieved it or replaced it to carry on preening. He selected pebbles of a particular size, and none of the other kea in his aviary used pebbles to preen. The authors describe this as deliberate self-care tool use, innovated out of necessity.

Tools in the wild

The stick episode in the Murchison Mountains is the strongest evidence of tool use by wild kea. Goodman, Hayward and Hunt, writing in Scientific Reports in 2018, explain that 720 trap-boxes were installed there from 2002 to catch stoats and protect the endangered ground-nesting takahe. Stick insertion probably began in late 2009 or early 2010. Video footage confirmed that kea were pushing sticks into the boxes. The authors describe it as the first documented case of habitual tool use innovated in the wild by a bird species previously known to use tools only in captivity, and they argue that the trap-boxes created an opportunity that the kea’s intelligence was ready to exploit.

The Contagious Play Call

Kea have complex play behaviour and a distinct play vocalisation. In 2017, Raoul Schwing and colleagues published a study in Current Biology testing what that call does.

Working with wild kea at Arthur’s Pass National Park, the researchers played recordings through a speaker: kea play calls, other kea calls, and the song of the South Island robin. Play calls increased the amount of play among both juveniles and adults. Speaking to National Geographic, Schwing explained that the birds were “immediately animated to play, but not by joining ongoing play already happening”. Instead, they “spontaneously started to play with the bird next to them”. The authors interpreted the call as a positive emotional contagion, a sound that spreads a playful mood in the way that laughter spreads among people, and compared it with the play vocalisations of rats.

What Kea Eat

Kea are omnivores with a broad menu. New Zealand Birds Online lists plants, fruit, nectar and seeds, insect larvae, tubers and huhu grubs, along with carrion from deer, chamois, tahr and sheep. A few kea in the Seaward Kaikoura Range prey on the nestlings of Hutton’s shearwater. A 2023 study in the Journal of Animal Ecology notes that kea play a role in seed dispersal.

That same study, by Wehi and colleagues, used stable isotopes in the feathers of 68 museum kea collected from the 1880s to the 2000s to reconstruct past diets. Nitrogen values in the feathers declined over time, which could indicate that kea in the early part of the record were highly omnivorous. Birds collected after 1950 showed a wider spread, with some individuals feeding almost entirely on plants and others mainly on animal food. The authors suggest caution, since part of the change may reflect shifts in the wider ecosystem.

Nesting in the Ground

Kea breed in natural cavities on the ground. New Zealand Birds Online lists rock crevices, hollow tree bases, spaces under tree roots, hollow logs and even a dirt cavity under the lip of an old river terrace. Kemp and colleagues, writing in the New Zealand Journal of Ecology in 2018, describe the nest sites as invariably on the ground and almost always below the treeline.

The usual clutch is 4 eggs, laid between July and January. The female alone incubates, for 22 to 24 days, and the chicks stay in the nest for about three months. Kemp and colleagues put the whole period from egg-laying to fledging at about 116 days, a long stretch of time for a ground nest to stay hidden from predators. Females can breed from 4 years old, and New Zealand Birds Online gives a known longevity of at least 22 years.

Stoats and the Ground Nest

Kemp and colleagues open their study by noting that the kea and other New Zealand forest birds are threatened by predation by introduced mammals. The Department of Conservation lists stoats as the main predator, killing adult kea, chicks and eggs, with feral cats doing the same. Possums damage nests and rats eat eggs. The department notes that kea are especially at risk because they nest on the ground and spend much of their time there looking for food.

Predator control makes a measurable difference. Kemp and colleagues followed kea nests in lowland rimu forest on the West Coast before and after an aerial application of 1080 bait over a 30,000 ha site, comparing it with an untreated control area. Before treatment, nest survival was 21% at the control site and 46.4% at the treatment site. After treatment it rose to 84.8% at the treated site and fell to 12.2% at the control. Aerial 1080 improved the odds of daily nest survival by a factor of 9.1, an effect the authors attribute mainly to the secondary poisoning of stoats. The Department of Conservation summarises the wider picture: where predators are controlled, about 70% of kea nests succeed, and without control success can drop to 10% or less.

Kea, Sheep and the Bounty

A reputation on the farms

According to New Zealand Birds Online, kea gained early notoriety among settler farmers for attacking sheep. The Dussex study notes that this reputation prompted a bounty scheme, and records that large numbers of kea were still being killed in the late 1940s, with nearly 7,000 birds killed over three years. New Zealand Birds Online puts the total at 150,000 birds shot between 1860 and 1970. Dussex and colleagues give a figure of over 150,000 killed between the late 1800s and 1971, when kea received partial legal protection. Full protection followed in 1986.

Despite the scale of the cull, the 2015 genetic study found no loss of overall genetic diversity associated with it, and the authors suggest that kea in remote areas may have escaped the worst of the hunting.

What modern farms show

Attacks on sheep, known as kea strike, still occur. A study by Reid and colleagues in the New Zealand Veterinary Journal examined 13,978 sheep after shearing on five high country farms in the South Island between 2012 and 2013. Injuries consistent with kea strike were found on 70 sheep, or 0.5%, with prevalence varying from farm to farm. Of the wounds, 80% were on the lower back, and the researchers described them as survivable. They concluded that kea strike was occurring at a low prevalence on the farms surveyed, and that its welfare impact should be balanced against the conservation status of the kea.

Lead: A Hazard of Curiosity

The kea’s habit of chewing and probing everything it finds exposes it to lead. Reid and colleagues reported in the New Zealand Journal of Ecology in 2012 that all 88 wild kea they sampled between 2006 and 2009 had been exposed to lead. Blood lead was significantly higher in kea from areas with permanent human settlements, such as Aoraki/Mt Cook village. There, 64% of birds had elevated levels and 22% had levels diagnostic of lead poisoning in parrots, while kea from remote areas stayed below the elevated threshold. Of 20 dead kea with tissue lead results, 11 had liver or kidney lead levels reported to cause poisoning in other birds. The authors identified roof flashings and lead-head nails on older buildings as likely sources, and observed bill marks on nails and flashing at Aoraki/Mt Cook typical of chewing parrots.

A second source has since been confirmed. In 2024, Buenz and colleagues used lead isotope fingerprinting on blood from 91 kea, published in Conservation Letters. About one-third of exposed kea had isotope ratios consistent with lead-based ammunition, which kea encounter when they scavenge shot carcasses. Working with tangata whenua, the Maori guardians of the species, the authors call for a shift to non-lead ammunition in kea habitat.

Conservation Status

Kea are protected in New Zealand and classified by the Department of Conservation as Threatened, Nationally Endangered. New Zealand Birds Online records that the national status was raised to Nationally Endangered in 2013. Internationally, BirdLife International moved the kea from Vulnerable to Endangered on the IUCN Red List in December 2017, as reported by 1News at the time.

Estimates of the total population vary. The Department of Conservation gives 1,000 to 5,000 birds, the same range quoted in the 2015 Dussex study. New Zealand Birds Online, in an account by Department of Conservation scientist Josh Kemp, gives a total of about 6,000 birds based on a conservative count of 4,000 adults. Local surveys show steep declines: in Nelson Lakes, adult density fell from about one female per 550 ha in 1998 to one per 2,750 ha in 2011.

New Zealand has a Kea Recovery Strategy running from 2024 to 2034, and the Department of Conservation reports that about 50 kea live in captivity in the country. The public has made its own vote: kea were named Bird of the Year in 2017 in the Forest and Bird competition.