Mōhua: the tiny forest bird back from the brink of extinction

Wednesday, 10 February 2021

On The Detail, Sharon Brettkelly meets two people who are doing their best to help achieve New Zealand's Predator-Free 2050 goal. (Video first published November 2019)

Mōhua were once widespread, with large flocks dominating the beech forests of the South Island. By the mid-1990s the population had crashed – down to just over a dozen in the Landsborough Valley.

But now, with a lengthy and expensive predator control operation, the tiny neon-yellow bird has bounced back and nature is rebalancing in the South Westland forest.

The songbirds – also known as yellowhead or bush canaries – are flourishing despite an explosion in predators in 2019.

That year saw a heavy seed fall – or “beech mast” – which attracts rodent and stoat plagues, who then turn on native species. The predators invade tree cavities where mōhua nest, killing eggs, chicks and adults.

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Mōhua were once widespread. Today they are estimated to number less than 5000.

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The spikes in predator populations threatened to undo two decades of pest eradication in the valley, which had allowed mōhua to slowly rebound.

But the country’s largest-ever predator control programme was launched in December 2019, protecting forest birds over 848,635 hectares of conservation land nationwide as they nested and reared their young over summer months. It will cost $81.2 million over four years.

Colin O’Donnell is DoC’s Principal Science Advisor.

Experts began counting birds in the Landsborough Valley in 1985.

“This was the last stronghold for [mōhua] in the whole of the West Coast,” said Colin O’Donnell, principal science advisor for the Department of Conservation.

“And, so we put a ring around it, to say this is a place to watch and be concerned about. But as we kept coming back, we found each year that we came that the numbers have dropped more and more.”

Predator control began a decade later, with valley-wide stoat trapping, and aerial 1080 drops over 30,000ha in beech mast years. The population was down to just 14 birds but has now increased 35-fold.

In the last monitoring expedition, in November, O’Donnell and his team registered 485 mōhua. That is up from 444 counted in 2018.

Six other native birds – tūī, korimako (bellbird), pīpipi (brown creeper), tītitipounamu (rifleman), riroriro (grey warbler) and kākāriki (yellow-crowned parakeet) – continued an upward trend.

Populations of kākā, ngirungiru (tomtit), pīwakawaka (fantail) and kererū remain stable in the forest. And two – tauhou (silvereye) and the migratory koekoeā (long-tailed cuckoo) – have declined.

It’s believed the tauhou are competing unsuccessfully with korimako and tūī for nectar.

The numbers of introduced birds have also dropped, probably due to greater competition from more native birds.

Native birds are now thriving in the alpine Landsborough Valley, where mōhua were once rare.

O’Donnell, and a team of bird experts, carry out dozens of bird counts at fixed points in the valley, providing an index of bird numbers.

For an intense five minutes, they count each burst of bird song in the area. Mōhua have a distinctive, melodious trill.

It is now DOC’s longest-running study, charting how birds respond to predator control and over the last 15 years the numbers have slowly begun to recover.

O’Donnell says the work was dispiriting, at first. But now mōhua have become the most common bird counted in the valley, up to levels seen before European settlement.

“It’s really exciting to see the sheer numbers of birds around us all the time because I work in other forests where it's really hard to find any sort of birds these days,” he said.

“It’s uncharted territory, in that we don’t know when native bird populations will peak, which is why it’s important to keep tracking progress at this site.”

O’Donnell recalls seeing flocks of up to 60 mōhua in the Hawdon Valley in the 1980s, feasting on insects.

“They were a crazy bunch of birds weaving through the canopy… they actually feed up and down the tree trunks, they run up and down the trees. They dislodge a whole lot of moss and rotten wood as well

“And they have this really strange tail, which has, holds them onto the tree, it has little spikes on the end of it.”

But although the bird is making a comeback, mōhua are still ranked as endangered on the international Red List of Threatened Species, just three steps away from extinction status.

The estimated total number is 5000.