Winner and losers - native birds in a pest-free sanctuary

There has been a huge change in the native forest bird community in Wellington’s Zealandia sanctuary in the past two decades.

New research shows there are far fewer piwakawaka (fantails), tauhou (silvereyes) and riroriro (grey warblers) than there were 20 years ago.

But there is nothing to worry about, according to Te Papa bird expert Dr Colin Miskelly. He says the small native species are still common around Wellington, while inside the sanctuary they have been outcompeted by thriving populations of rare endemic birds such as kaka, tūī, tīeke (saddlebacks) and hihi (stitchbirds).

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Nearly two decades ago, Zealandia became the first predator-free fenced sanctuary on the New Zealand mainland. Back then, its bird populations were nothing to write home about – a mix of common small natives and introduced species, such as blackbird, starling and chaffinch, that were common across Wellington city.

As the idea of a predator-free sanctuary surrounded by a protective fence began to grow in the mid-1990s, Colin Miskelly had an idea: a citizen-scientist project to monitor how the local bird community might change over time.

The first three years of bird counts took place from 1995-1998, just before the fence was built.

“It took a bit of convincing local bird watchers, members of Birds New Zealand, to come into a patch of scruffy old bush on the edge of Wellington city to count common birds. Because that’s all that was here at the time,” says Colin.

“We were literally counting hundreds of silvereyes, blackbirds and chaffinches. But it was about laying the groundwork so we could measure the changes once the fence was put in.”

After the predators were removed - and before rare species such as tīeke began to be reintroduced - the bird counters returned for another three years of surveys.

What they found was not what they had expected.

“The really intriguing result was that not much changed,” says Colin. “You’d think that if you got rid of the predators everything would increase – but that didn’t happen.”

“There was one species, which was tūī, that did really well – they started increasing. But pretty much everything else that was here before the fence didn’t change much when we got rid of all the mammals.”

Colin says this suggests that common, robust species – such as tauhou and piwakawaka – can cope with predators such as rats much better than we have assumed. He thinks it is more likely that their populations are limited by food supply.

“So things like fantails, when you get a bad winter, with stormy conditions and not many insects flying, that’s really what affects [their numbers].”

In 2013, the bird counters returned to Zealandia for another round of counts – and by this time bird populations in the sanctuary had changed significantly.

Successful reintroductions at Zealandia have included pōpokotea (whitehead), toutouwai (robin), kākā, kākāriki, tīeke, korimako (bellbird) and hihi, while the local tūī and kererū populations have bloomed.

“It’s marvellous,” says Colin. “You think we’re only a couple of kilometres from the Beehive, and you can just walk into this little patch of forest and nearly everything you see and hear around you are these ‘deep’ endemic New Zealand birds. It’s a real success story.”

Colin says that piwakawaka, tauhou and riroriro are still present in the sanctuary, but in much smaller numbers than before. The main reason for this is competition with endemic species that, evolutionarily speaking, have been in New Zealand for a very long time and are well adapted to our forests.

The small natives are relative newcomers that arrived here from Australia. They are generalists that are better adapted to survive in the presence of rats and stoats, and thrive on scrubby bush margins rather than dense native forest.

Colin says that one advantage the recent natives and introduced bird species have is that they are fast breeders that can produce a number of clutches each breeding season. Even if they lose some chicks to predators, some will survive.

Colin is not at all concerned about the decline in piwakawaka, tauhou and riroriro numbers. “It’s a Zealandia-specific effect,” he says. “They are still really common around Wellington city.”

“Changes in the forest bird community of an urban sanctuary in response to pest mammal eradications and endemic bird introductions” by Colin Miskelly (2018) is published in the Birds NZ journal Notornis.

More stories about native birds in Wellington

Native birds doing well in Wellington reveals the good news from the regular bird surveys conducted by Greater Wellington in parks and reserves  across the city.

Predator-free in the city looks at the work of a community group which is trapping predators in a bush reserve next to the Zealandia sanctuary.

Smart kākā – can you teach an old parrot new tricks? looks at Victoria University of Wellington research into how smart Zealandia kākā are.

In Goodie goodie – bird watching with Bill Oddie we join the ex-Goon and bird watching enthusiast for a wander around Zealandia.

Bird brains – measuring the IQ of bush robins investigates the smarts of Zealandia’s toutouwai.

In Kākāriki in the capital we meet Zealandia’s red-crowned parakeets.