Weka can be delightful and entertaining, but their opportunistic appetite for other birds’ eggs and chicks can cop them some controversy. They’re not always welcomed, even though their own numbers are threatened.
As opportunistic feeders, weka are not always welcome in ecosanctuaries or places where conservation efforts focus on restoring highly threatened birds, lizards, and insects.
New Zealand was never predator free
Aotearoa didn’t evolve without predators – it evolved without mammalian predators. Native predators such as weka, kāhu (harrier hawk) and kārerea (falcon) were once part of New Zealand’s balanced ecosystem.
That balance was disrupted by the arrival of introduced predators. In some situations, the omnivorous eating habits of weka now appear to be one more pressure on species already struggling to survive.
Scientists Joanna Carpenter, John Innes, Jamie Wood, and Phil Lyver from Manaaki Whenua – Landcare Research (now Bioeconomy Science Institute) investigated what we know about the roles of weka in both past and present ecosystems.
“Although the important role of predators in ecosystems is now widely acknowledged, restoring predator populations is still controversial, because predators can have deleterious impacts on other valued species.
“Weka are a textbook example of this conflict, being a threatened species that sometimes preys on other threatened species.”
What harm do weka really cause?
Evidence for individual-level impacts of weka on native vertebrates is abundant, yet compelling evidence for population-level impacts is scarce.
Weka have been recorded destroying the eggs and chicks of ground nesting birds, including kiwi, Fiordland crested penguin, tītī, southern Buller’s albatross, Chatham Island oystercatcher, takahē and whio. They can also kill the fledglings and chicks of birds such as tīeke, robins, tūī and kākā. Weka also consume lizards and introduced frogs.
“Weka predation is believed to have driven declines of tītī on Kāpiti Island over the last 30 years, and a weka-proof fence has been constructed to protect the last breeding colony. However, a population of Westland petrels on the South Island has high adult survivorship and fledging success in the presence of weka.”
Invertebrates make up a large part of weka diet, but the impacts of this feeding are often not well studied, except for some of the larger, more charismatic species.
A study in the 1980s recorded highly variable levels of weka predation on Powelliphanta (land snail) species and concluded that although native predators like weka should not, by themselves, seriously threaten Powelliphanta species, weka predation combined with introduced predators, habitat modification, and habitat loss could have serious consequences.
“Weka are also more likely to reduce prey populations when the prey species is already limited or has been limited by other factors, such as predation by introduced mammals. Habitat loss and degradation may also play a part, as the loss of habitat that acts as refuges from predation could amplify the impact of weka on prey populations. For example, browsing by ungulates removes understorey and ground cover, exposing Powelliphanta snails to weka and other predators.”
But because they do prey on other threatened species, weka have been deliberately eradicated from at least 11 offshore islands, say the authors, including some where they occurred naturally (e.g. Entry Island, Fiordland).
“They are frequently excluded from mainland restoration projects despite their historical presence at ecosanctuary sites, mainly due to fears that weka will preclude opportunities to introduce other threatened fauna.”
Overlooked benefits of weka
Weka have also been observed killing introduced vertebrates, including rats, mice, mustelids and rabbits (watch this video to see a weka chasing a stoat).
Whether weka could meaningfully suppress introduced mammals hasn’t been tested.
“It would be particularly useful to know whether weka could suppress mouse populations in the absence of other mammalian predators. Mice are present in most fenced ecosanctuaries in New Zealand and can reach high densities once other mammalian competitors and predators have been removed. Burgeoning mouse populations are also likely to become an increasing problem across New Zealand if Predator Free 2050 is successful, as this initiative involves removing several introduced species that currently suppress mouse populations. If weka are capable of suppressing mice, they could become a useful tool for both ecosanctuaries and the Predator Free 2050 initiative.”
As omnivores, weka are involved in several ecological processes in New Zealand’s ecosystems besides predation.
Their large gape and fruit-eating habits make them an important seed disperser, especially for species such as hīnau, which may rely on dispersal by flightless birds. Remarkably, they can retain seeds in their guts for almost six weeks (the longest avian seed retention times ever recorded), which gives them the potential to carry out important long-distance seed dispersal. They also scavenge on carcases, removing carrion and potentially reducing disease transmission.
Weka are also valued by Māori with customary harvest of weka offering potential socio-ecological benefits. Historically, they were harvested for food, feathers for korowai, oil for medicinal use and as a means of rat suppression.
So, where does this leave weka?
The importance of native predation is usually overlooked in restoration scenarios, the authors say.
“Weka were historically a part of many ecosystems across the New Zealand mainland,” the authors conclude, “so projects truly dedicated to ecosystem restoration should welcome their presence as an archetypal generalist that carries out many ecological processes.”
That said, reintroductions of weka to mainland sites will probably fail unless intensive control of mammalian predators is also implemented.
Indeed, North Island weka are the most translocated bird in New Zealand conservation history, with 79 recorded translocations, yet only 8 (6 of which were to islands) have been successful, probably due to a combination of predation by exotic mammals and widespread dispersal by released birds.
If weka aren’t always welcome in our ecosanctuaries, perhaps we can at least welcome them in our suburbs.
“Weka are engaging and charismatic, although they can have several nuisance behaviours, and can live in human-dominated, peri-urban and rural landscapes. Therefore, restoring weka also provides an opportunity to restore lively relationships between people and native birds in the places where most people live.”
‘Lively relationships’? The weka will surely be up for that!
The full article is published in the New Zealand Journal of Ecology and is freely available online.
