It’s a tale of two halves for rock wren/tuke with their fortunes hanging on effective predator control, the latest South Island-wide survey shows.

The Department of Conservation monitoring programme, now in its third year, tracks populations of this small alpine bird to see the effects of management.

Each January six field researchers set out in Fiordland and work their way north to Nelson. In teams of three they walk or fly into 22 mountain sites and camp in alpine basins to survey over a few days.

At each site they count rock wrens/tuke along 20 fixed lines (each 250m). It’s exacting work spotting the small birds as they hop and flit among the boulders and listening for their high-pitched calls. They also survey for kea while there.

DOC research lead Tristan Rawlence says results show rock wren/tuke numbers are increasing at the 12 sites where predators are regularly suppressed but are gradually declining where they’re not.

“We’re seeing good numbers of rock wrens in areas where we’re keeping predators down using 1080 and traps, and they are trending upwards.

“In contrast, at the 10 sites where there is no predator control, rock wren numbers are low, and populations are continuing to decline or have locally disappeared.

“Overall, there are three times more rock wrens in predator control areas compared to unmanaged sites,” says Tristan Rawlence.

Rock wrens/tuke, which are found only in the South Island, are generally doing better in southern areas and west of the main divide, where there is less pressure from predators compared to the east.

In Westland, where podocarp and hardwood forests have fewer predators compared to beech forest, rock wrens/tuke have held out at unmanaged sites but are trending downwards. It’s hoped planned predator control operations in some of these areas will arrest this decline.

Populations in Kahurangi National Park that were affected by the 2019 mega beech mast and resulting stoat plague have rebounded.

Tristan Rawlence says long-term monitoring is important to see how rock wren populations respond to different predator control techniques and the effects of climate change and other environmental factors.

A warming climate will allow predators such as rats to better survive in alpine environments. This has the potential to squeeze rock wrens out of areas such as Kahurangi National Park, which lacks higher mountains for them to seek refuge in.

Rock wrens/tuke live year-round in the harsh alpine environment and are thought to semi-hibernate for periods in winter. No bigger than a silvereye, they are weak flyers and nest on the ground, making them easy prey for rats and stoats.

The monitoring programme is part of DOC’s National Predator Control Programme, which protects wildlife and forests across public conservation land.

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