Aotea’s Kererū: a Taonga Species and Essential to Forest Survival

KATE WATERHOUSE with JOHN OGDEN (our Science Advisor)

If you park your car in the shade of the large pūriri at Harataonga you are likely to encounter the main seed distribution mechanism of many of our forest trees — the digestive tract and droppings of the kererū. Kererū and kōkako  are the only birds that are able to swallow the large fruits of some of our most important forest trees—the taraire, mīro, matai, tawa, karaka and pūriri.  If there are no kererū, these trees are unable to disperse their seeds away from the parent tree.  The kererū digests the coating around the seed inside the fruit and then distributes the seeds in its droppings all over the forest.

Introduction

According to the Kererū Discovery Project, kererū (Hemiphaga novaeseelandiae) eat the fruit, leaves, twigs, buds, and shoots of over a hundred native, and fifty exotic, shrubs and trees. In an intact ecosystem, a kererū might expect to live for 20 years or more, but they may survive, on average, for around five years before falling victim to pests, cars or collisions with windows. This represents a huge loss to the breeding population. Increasingly, urban gardens are recognised as critical to the survival of kererū, and every Barrier gardener with a guava tree knows they’re ripe when the kererū move in. A garden where rats are controlled is even more attractive to kererū.

Kererū are of great significance for Māori

Te iwi Māori have an enduring relationship with kererū that is a spiritual connection, transcending mere sustenance(1). They were one of the most important birds in the forest and their significance cannot be adequately covered here. To understand something of this, remember that there are early colonisation mainland records of flocks of 300 kererū.  The noise and effect of the birds moving in the forest of old would have been very powerful. Feathers had particular uses and there were particular traditions and times that governed hunting of them. Provision of kererū as food demonstrated capacity as kaitiaki, to care for the current and future prosperity of the environment. Such were their importance that kaumātua or tohunga were tasked with holding the mana and mauri (life force) of the kererū. Decline in kererū populations has been attributed to government usurping the mana for the kererū and forest from Tuawhenua. Dying kuia or kaumatua are said to have called for a final meal of kererū to help them on their journey to the afterlife. Many iwi now have rāhui in place to protect kererū from hunting. Other names for kererū include kūkū and kūkupa, all similar to the quiet cooing sounds they make while at roost.

E koekoe te tūī, e ketekete te kākā, e kūkū te kererū
“The tūī chatters, the parrot gabbles, the wood pigeon coos.” Or, it takes all sorts…

Why did kererū decline on Aotea? 

At Okiwi the forest floor under kererū roost trees is always carpeted with seedlings of nikau, kohekohe, pūriri, taraire and pigeonwood. But these trees are exceptions rather than the rule these days. Kererū are just holding their own on Aotea, and given their role in forest ecology and regeneration, they need our help to recover. The loss of so much of the island’s mixed coastal broadleaf and podocarp forests that sustained kererū, and more recently, feral cats and the arboreal ship rat, have decimated their numbers on Aotea in just the last 150 years.  Predators have a double impact, preying on eggs and chicks in the nest as well as competing with kererū for fruit, seeds, leaves, and berries.  

Gone are the days when flocks of 30 birds rose and swooped in the valleys of Tryphena, Awana, Okiwi or Motairehe. You might see a flock at Windy Hill if you’re lucky, but otherwise a single kererū at roost, a pair of kererū feeding on kōwhai during winter (they are monogamous), or perhaps a male engaged in his elegant dives and swoops near the nest, are what we think of as “normal”. It most definitely is not, and the fact that kererū were a major food source for Māori and for early settlers tells us how abundant they were before the island’s forests changed. 

Dr John Ogden had been analysing Aotea’s bird data for the upcoming Status of the Birds, Aotea Great Barrier Island report(2) (see below), a comprehensive review of bird data, that builds on the 2010 Great Barrier Island State of Environment Report(3), which he co-edited with Liz Westbrooke. John says: “I would say, tentatively, that kererū are slowly increasing on Aotea.” 

Kererū on Aotea (State of our Birds Review) 

The kererū is a large endemic pigeon, which was formerly very widespread and abundant in New Zealand and was hunted for food by both Māori and Europeans. Its decline on Aotea Great Barrier Island mirrors that seen elsewhere in the country. Since kererū were protected (1921), there is evidence of a slow recovery throughout New Zealand, but illegal hunting, the species’ low fecundity, and vulnerability to predation of the single egg and chick puts it at risk. The species appears dependent on conservation efforts, and introduced food plants, for its gradual increase.  

The status of kererū is hard to assess because, except when performing its characteristic display flight, it usually remains well hidden in the forest canopy and makes little noise.  In contrast to former times, flocks of more than ten birds are rarely seen on Aotea. Bell & Braithwaite (1964)(4) noted that in 1957 single birds were often seen in the bush, and a flock of 23 was seen flying high near Tryphena.  In the same year the number of kererū on Rakitū was estimated at between 25 and 50. It was recorded as “common in forested areas” on Rakitū by Bellingham et al. (1982)(5) but the largest flock seen was “up to 11 birds”.  The overall impression gained from locals involved in the 2006-2008 GBICT surveys, was that kererū were in decline. At that time average frequencies of 20-25% were recorded in areas where there was some mature forest, but much lower frequencies in mānuka/kānuka bush (5%) and open paddocks (1%).  However, since then, counts at Windy Hill(6), and the new, annual, Aotea Bird Count (2019 onwards)(7), (8) suggest a gradual increase, at least in areas where rat management is occurring. The species was recorded on 14 of the 16 Aotea Bird Count transect lines in 2019, with an estimated mean density of 1.02 individuals/Ha in the surveyed areas(7). This is similar to the estimated density currently in the rat-managed area at Windy Hill(6).  At the currently estimated density of 1.0 kererū/Ha in indigenous forest areas, the population on Aotea would be c. 950 birds at most.

The kererū is regarded as a ‘keystone species’ of the Oceanic Temperate Forest ecosystem because of its role in dispersing the seeds of large-fruited tree species, such as miro, tawa, taraire, pūriri, and karaka, which are too large to be dispersed by other birds.  Kererū will congregate in flocks to exploit such a resource; the largest reported flock on Aotea comprised c.50 gathered at Windy Hill in 2021 to exploit the abundant taraire fruit(6). Kererū may be particularly important on Aotea/Great Barrier Island, where remnant patches of mature forest containing these species cannot easily extend from gullies or lowland sites to replace the ubiquitous kānuka stands, without such ‘up-hill’ assistance. 

What is preventing faster kererū recovery on Aotea? 

John Ogden suspects that that while periodic food shortages are a factor limiting kererū recovery on Aotea, the success of kākā, also shown in the Windy Hill data(6), is probably also having an inhibiting effect. As he concludes in the State of Our Birds report(2):

“Like toutouwai/robins, kākā usually increase after pest eradication or control, while kererū do not (Miskelly et al. 2021)9. This ‘correlation’ is unlikely to be due to direct responses to predation or competition between the two species… They co-exist in many forested areas of Aotearoa, but the competitive balance during times of food shortage might favour kākā, many of which migrate off-island in winter. Lower frequencies of kererū in the presence of increased numbers of tui and kākā were observed by Ogden (2010)(10).  He speculated that food resources could limit bird numbers via competitive hierarchies if predation ceased to be the main factor. Whatever the causes, the numerical changes at Windy Hill, following twenty years of pest management, fit the general picture following reduction or elimination of rats: increase for kākā, stasis for kererū.”

Mick Clout, Professor of Conservation Ecology and a New Zealand expert on kererū, when queried on this issue, suggested kererū’s slower recovery, and slow population growth, is due mainly to their lower reproductive rate, rather than other factors such as competition for food from kākā and tūī, which appear also to be starting to recover on Aotea(11). 

A recent Predator Free NZ Trust article on the national decline of kererū described why kererū reproduction is low(12). “Kererū raise one chick at a time, incubating their single egg in a twiggy nest for about 4 weeks. The parents feed the chick a protein-rich ‘milk’, which they secrete from their crops, adding partially-digested fruit to their diet after a couple of weeks. The chick leaves the nest at 30-45 days old, but the parents continue feeding them. So in total the single egg/chick is in the nest for 60-75 days. That’s a lot of opportunity for nest-raiding predators.”

Mick Clout(11) points out that “in comparison, tūī lay clutches of two to four eggs, incubate them for 12-15 days and the chicks can fledge two weeks after hatching. Kākā on the other hand lay between one and seven eggs; usually about four. Tūī and kākā (therefore) have the potential to increase in abundance more rapidly than kererū. So, perhaps it's not just a matter of (increased) competition for food resources, but the lower reproductive rate of kererū being a key factor.” 

A highly relevant paper, which the Predator Free NZ article draws on is that by Carpenter et al. (2021)(13). In summary, it confirms that kererū population growth is generally slow, and that many populations (especially in the South Island) have declined over recent decades. Their analysis of all available data showed that predation is the cause of most nest failures and deaths in kererū. Of the 397 nests in the study, a massive 50% suffered ship rat predation of eggs (and remember that ship rats are the main predator on Aotea). Stoats (not present on Aotea) were the main predator of chicks, and feral cats the main predator of adult kererū. 

Carpenter et al. (2021)(13) go on to state that kererū may be particularly susceptible to the effects of food limitation because their breeding is highly variable and appears to be correlated with food supply. Lack of food can increase mortality of adults and fledglings through starvation.  But it can also have an indirect effect by increasing predation risk (because hungry adults are at the nest for less time), and increasing susceptibility of chicks and adults to parasites and disease. 

Finally, they suggest that “in warmer, forested sites (like Aotea), where food is less limiting, and median pest densities are highest, kererū should be decreasing most rapidly over time.” This is chilling, given that ship rat densities on Aotea are already amongst the highest in Aotearoa(14),(15). But this may be offset on Aotea by more frequent breeding, if Aotea birds are like kererū in Northland, which the authors state may lay eggs at any time of year except when moulting.

So why is there good news for kererū at Windy Hill?

In the study by Carpenter et al. (2021)(13), unpublished data from Aotea, and in particular Windy Hill Sanctuary, was not included in their analysis. It shows a different trend — a slow increase in kererū numbers at that site over time. Only Tāwharanui Sanctuary, north of Auckland, has had similar results. This is a vindication for Judy Gilbert, Sanctuary Manager, and the many people who have worked to keep rat densities low and feral cats out of the area for two decades. Interestingly, this positive change on Aotea is  supported by another conclusion Carpenter et al. (2021)(13) came to — that “the low reproductive rate of kererū means that there may be a considerable lag before increased numbers of kererū are observed in sanctuaries.” 

Complicating the picture, kererū are highly mobile, with home ranges as large as 31,732 ha according to Carpenter et al. (2021)(13), and “an excellent ability to cross habitat gaps”. On Aotea, this means kererū could, in theory, move all over the island (which is about 28,000 ha), to and from Rakitū, and to both Moehau and Hauturu/Little Barrier Island, in search of food. 

Because of this, Carpenter et al. (2021)(13) suggest that interpretation of the impact of predator management is difficult on the mainland and near-shore islands like Tiritiri-Matangi. “The kererū detected at non-treatment sites (ie: with no predator control) may be the same individuals detected in sanctuaries. Alternatively, increased numbers of kererū at a managed site may simply reflect immigration into the site rather than a net increase of kererū in the wider landscape.” 

We  will soon know more. In 2022, trend data from the Aotea Bird Count will be available for the first time, showing kererū presence over three years across at least 16 sites at a point in time (early December every year).  Then we may see whether the 21-year trend at Windy Hill is mirrored across the wider landscape of Aotea, as predator management effort increases on both public and private land. 

A taonga and a sentinel 

In correspondence over this article, John Ogden(16) pointed out that “we don’t really know what the outcomes will be for our forest ecosystems once predators are removed, but that there is abundant evidence that the long-term effects of pest eradication  are good for all endemic species. But it is probably the case for kererū, that due to their low reproductive rate, other species such as tūī and kākā may benefit more rapidly”.  John posits that the new balance will take several generations (many decades) to stabilise.  

Spending time out in the remnant halls of the broadleaf and podocarp forest that used to clothe Aotea, the bush itself tells me that we will need kererū more than ever in the future. As our climate warms, it’s clear that species such as taraire and pūriri are becoming more stressed and even dying off in hot dry summers, such as we have had in 2019 and 2020.  Kererū are needed to protect and restore Aotea’s physical forests and also the mauri of those places. Who of us does not wish to see flocks rising white against the hills and to hear the whoosh as they swoop and flare on the updraughts in our valley, feeling the air move as they pass. 

So what can be done to bring more kererū back? 

It’s pretty simple. Support the recovery of more non-kānuka dominated forest and reduce predation from feral cats and rats, by eliminating them from Aotea. Predators are the handbrakes on kererū recovery, which in turn is slowing forest recovery. Kererū are a sentinel, taonga species and a critical reason for more active protection of mature forests and pursuing the vision of a predator free Aotea.  It will take decades to see a return to abundance of the past, but in the meantime, controlling rats and feral cats through projects such as Tū Mai Taonga (see interview with Jo Ritchie), and protecting the pockets of old forests that remain on Aotea are what we must do.  

At Okiwi, we’ll be planting a few hundred more of those fat berried miro, tawa, matai, taraire and pūriri, for the descendants of the pair of kererū that own the kōwhai round our house. And perhaps we need a few more guavas in the garden too.

References:

1. Timoti P, Lyver PO’B, Matamua R, Jones CJ and Tahi BL (2017). A representation of a Tuawhenua worldview guides environmental conservation. Ecology & Society 22 (4): Article 20. https://doi.org/10.5751/ES-09768-220420

2. Ogden J (2022). Status of the Birds, Aotea Great Barrier Island (unpublished).

3. GBICT 2010. Great Barrier Island State of Environment Report. Pp 187. Unpublished report by J. Ogden and E. Westbrooke. www.gbiet.org

4. Bell, D.B. & Braithwaite D.H. 1964. The birds of Great Barrier and Arid Islands. Notornis 10: 363-383. 

   and Braithwaite (1964).

5. Bellingham, P.J., Hay, J.R., Hitchmough, R.R. &  McCallum, J. 1982. Birds of Rakitu (Arid) Island. Tane 28: 141-147.

6. Windy Hill Rosalie Bay Catchment Trust (2021). Trends in Bird Abundances at Windy Hill 2000-2021. Unpublished Report by J. Ogden.  

7. GBIET 2020. Aotea Bird Count. Results of the December 2019 survey.  Great Barrier Island Environmental Trust. Unpublished report by Selena Simmonds, July 2020. www.gbiet.org

8. GBIET 2021. Aotea Bird Count. Results from the December 2020 survey.  Great Barrier Island Environmental Trust. Unpublished report by Quinn Asena, August 2021. www.gbiet.org

9. Miskelly CM, Greene TC, McMurtrie PG, Morrison K, Taylor GA, Tennyson AJD and

   Thomas BW (2021). Species turnover in forest bird communities on Fiordland islands following predator eradications. N Z J Ecology 45:1-18.

10. Windy Hill Rosalie Bay Catchment Trust (2010). Windy Hill - Rosalie Bay Bird Counts December 2009. Unpublished Report by J. Ogden. 

11. Clout, M. personal communication, December 2021.

12. Newsletter of Predator Free NZ. Nov 11, 2021. What factors are limiting kererū populations? Unpublished report by Kate Guthrie. https://predatorfreenz.org/research/kereru/

13. Carpenter et al (2021) Factors limiting kererū (Hemiphaga novaeseelandiae) populations across New Zealand. N Z J Ecology 45:3441. Available from kererudiscovery.org.nz (accessed 1/1/22)

14. Russell J, Bodey T, Peace J and Veale A (2018). Rakitu Island terrestrial survey January 2018. Unpublished report.

15. Russell J (2018). Ratting on Rakitu.  Unpublished Report, February 2, 2018. https://aucklandecology.com/2018/02/02/ratting-on-rakitu/

16. Ogden, J. personal communication, December 2021.