Mohua are bright yellow forest birds - but despite their eye-catching plumage, they can be tricky to spot flitting high in the forest canopy. Claire Concannon visits the Makarora mohua population, where a team of conservationists and scientists are testing acoustic machine learning to identify individual birds.

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A 'rat plague' has come to the beech forest near Makarora, and Jo Tilson is worried for the little population of mohua who call this area home.

The 'bush canary'

In the 1800s mohua were one of the most abundant forest birds, found in different forest types. Their bright yellow heads (hence their common English name, yellowhead) and yellow-feathered bellies led them to being dubbed the 'bush canary' by European settlers.

But - and you know this story - forest clearance and introduced predators reduced their population and range significantly. These little songbird insectivores nest in holes in trees, and so are vulnerable to stoats and ship rats. Some mohua have been transferred to predator-free offshore islands where they are doing well, but on mainland New Zealand there remains just a few small, scattered populations living in South Island beech forest.

One of these can be found just north of Makarora, near Cameron Flat, in the silver beech forest that lines the west side of the Makarora River as it runs alongside State Highway 6 through the Haast Pass.

It takes a village

Jo works as a biodiversity coordinator for Southern Lakes Sanctuary, focused on species and predator monitoring for the Mātukituki and Makarora hubs. Southern Lakes Sanctuary is essentially an umbrella organisation that has wrapped around existing conservation groups to enable a regionwide effort.

In this area, that includes a highly motivated group of 60 or so trapping volunteers from the Central Otago Lakes Branch of Forest & Bird. With the first trap lines established in 1998, the group has been "sustainably harvesting predators," as volunteer Mo Turnbull puts it, for more than 25 years, to help the local mohua population hold on.

But it's not about dead rats, Mo says, it's about live mohua. So how are they doing?

Cryptic in the canopy

Transects run by the Department of Conservation and the sanctuary across the years indicate that the mohua numbers in the area are declining. But without a complete census it's hard to have the full picture of what's going on. …

Go to this episode on rnz.co.nz for more details