A recent dip in mohua/yellowhead numbers in a remote South Westland forest shows how closely its fortunes are tied to predator control.
Featured on our $100 banknote, mohua are classed 'At Risk' with populations in decline apart from a few South Island mainland sites and pest-free islands.
The Department of Conservation has been monitoring forest birds in the Landsborough valley annually for the past 28 years to measure the effects of predator control. It's the longest-running project of its type.
When surveys began in 1998, only 14 mohua were detected in the 350-ha study area. Without intervention this bright yellow forest bird was predicted to be lost from this site, as it has been from many other South Island forests. Instead, the Landsborough became a stronghold as mohua numbers peaked at more than 500 in 2022. Mohua became the most common native bird in the area.
However, the past two years of monitoring has shown a temporary dip in mohua numbers – dropping 25 per cent in 2024, then rebounding by 21 per cent in 2025.
DOC National Predator Control Programme Manager Peter Morton says this fluctuation shows how sensitive mohua are to predators.
"In 2023 mohua took a hit from a surge in rat numbers fuelled by local beech forest seeding before a predator control operation was run in January 2024.
"Mohua roost and nest in holes in trees and are helpless when rats invade.
"Last November, mohua numbers were on their way up again, with rat and stoat numbers reduced to very low levels.
"This shows how critical well-timed predator control is to mohua survival – if we misstep, this sensitive species can go backwards disturbingly quickly.
"We're hoping to see mohua continue to recover from this recent blip when we monitor birds again this coming spring."
Over the past six months DOC has carried out aerial 1080 predator control operations over 600,000 ha in priority South Island conservation areas, including the Landsborough. The programme aimed to prevent rodent plagues impacting native wildlife following this year's widespread beech mast.
The Central Otago branch of the New Zealand Deerstalkers Association (NZDA) has run a trap line in the lower Landsborough for the past two years, helping to stop stoats and rats from reinvading the valley. Their dedicated volunteers check the traps each month and have reported a noticeable increase in birdlife in the area due to their work.
"Not everyone will get to hear the amazing birdlife in the Landsborough valley, but like the volunteers from the Central Otago NZDA, people can help restore nature by setting traps in their backyard or joining a local trapping group. These acts of naturing all help as we work towards Predator Free 2050," says Peter Morton.
In the Landsborough, predators are kept low year-round by traps along the valley floor and aerial 1080 operations when forest seeding causes rat numbers to rise.
