Ship rats hit rare yellowheads

By ANNE BESTON

Global warming may be making life even more difficult for one of New Zealand's rarest native birds.

Mohua or yellowhead (Mohoua ochrocephala) numbers have fallen dramatically because of the ship rat, a lethal predator that is invading new areas and catching the Department of Conservation by surprise.

Over the past few years, ship rat numbers have swelled to unprecedented levels in South Island alpine areas, one of the last refuges of mohua on mainland New Zealand.

"We've had a couple of mild winters in the south and ship rats are now abundant at higher altitudes," said Andy Roberts, who heads DoC's mohua recovery programme.

"They are moving up to places where mohua are. You have to wonder about climate change or global warming, and we're asking the question as much as anyone else."

The mohua population has dropped 70 per cent in just two years, even in areas where DoC has carried out intensive predator control.

The distinctive yellow-headed birds are larger than a sparrow but smaller than a blackbird. They are found only in the South Island.

They were already struggling because of stoats, but DoC staff felt they were slowly getting on top of that problem - until the ship rats arrived.

"The rats are very good climbers. They'll climb right up into the canopy and that's where mohua are," said Mr Roberts.

Abundant beech seeding for the past two seasons has also contributed to the dramatic mohua decline. Mice numbers have swelled, and with plenty of mice around, the stoat population has also boomed.

"It's the kind of thing that usually happens once every 20 years but for mohua it's been a double whammy," Mr Roberts said.

One carefully nurtured mohua population at Mt Stokes, in the Marlborough Sounds, had risen from just 20 birds to almost 100. That population has now been wiped out, mainly because of rats.

Since 1995, the department has begun establishing mohua on islands in a last-ditch attempt to save them from extinction. But Mr Roberts said the real battle was being fought on the mainland.

"We want them where people can see them."

Some people had urged the department to use 1080 against the rats but Mr Roberts said that was a tough decision that the department was not yet ready to make.

"It may be more a case of dealing more holistically with predators in areas where we have more than one species we want to save," he said.

"We need to get more bang for our bucks and that might mean some populations fall by the wayside. But we need to concentrate on key sites - we're on a learning curve.

"Extinction is a nasty word," said Mr Roberts. "We can stop it but they will certainly never be as widespread as they used to be."

Department of Conservation

Kiwi Conservation club

Herald feature: Environment