Crafty kea have once again proven their status as New Zealand's smartest birds, this time being caught on camera using sticks to set off stoat traps.

The puzzle started in Fiordland, where rangers had noticed solid wooden stoat traps had been triggered, often with sticks left behind.

Mat Goodman, 23, picked up the mystery while working with a documentary team in Fiordland and wound up investigating further as his final project for a University of Canterbury degree.

"It was obvious that it was kea really, but nobody had seen it happen," he said.

So he set out to catch the culprits in action using motion-activated cameras.

But the alpine parrots did not take kindly to surveillance. "I had about four cameras that were ripped to bits by kea," Goodman said.

After some trial and error Goodman found a setup that enabled his camera to survive several weeks until he could return to rescue it.

It was that camera which caught the incriminating evidence. Over two hours, what appeared to be a single bird "meddled" with a stoat trap, collecting and testing sticks until it found one that set the trap off. Goodman said it appeared the bird was "whittling" sticks to make them the right size, pulling off any twigs before testing them.

If a stick did not work, the bird tossed it away in a manner that seemed remarkably like frustration.

Goodman sent the footage to Auckland University psychologist Dr Gavin Hunt, who has studied "tool use" by New Caledonian crows.

Hunt agreed, it seemed the bird caught on camera was fashioning sticks to set the traps off which would be classed as tool use.

The Department of Conservation (DOC) had already modified the stoat traps several times to try to stop them being set off.

After the last modification Goodman said it took a few months before rangers started seeing the traps set off again.

"It's probably very frustrating for DOC," Goodman said. Particularly as it appeared the only reward the birds got was "a very loud bang".

Though the traps were baited with food, the bait was almost always left behind after the trap had been triggered.

"So that's going to annoy a few people I think," Goodman said. "If it is the bait then there's a possibility to try to change bait."

But Goodman thought the annoyance was worth it for the crafty birds which he said were an "asset to New Zealand".