A New Zealand scientist has found yet more evidence to suggest New Caledonian crows might just be the smartest birds on the planet.

University of Auckland researcher Dr Alex Taylor has been working with the clever species for years, showing how they're able to use tools to solve extremely complex tasks.

Now Taylor and colleagues have revealed crows can recreate tools from memory - something that could allow them to improve them over time, or help other birds with their own.

Remarkably, the skill is rarely associated with non-humans.

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The crows are known to make basic stick tools, hooked stick tools and barbed tools torn from the leaves of plants.

But until now, it hadn't been clear if they could learn tool designs from other crows and if these designs improve over time.

"We were trying to understand why the tool designs of New Caledonian crows appear to have increased in complexity in the wild, when these birds don't appear to copy each others' behaviours, teach each other or have language," Taylor explained.

"Put simply, how could these birds faithfully copy their tool designs from one generation to the next and then improve them when they don't appear to copy?"

The research team, led by Sarah Jelbert of the University of Cambridge, sought to answer that question by training eight crows to drop differently sized pieces of paper into a vending machine to retrieve rewards.

Only pieces of a particular size were rewarded.

A New Caledonian crow inserts a card tool into food dispenser. Photo / Sarah Jelbert
A New Caledonian crow inserts a card tool into food dispenser. Photo / Sarah Jelbert

Once the birds had been trained to recognise which one got them a pay-off, the researchers provided them with a large piece of card, but no physical templates of the previously rewarded paper sizes.

They were amazed to watch the crows tear up the card to form items similar in size to the pieces of paper that they'd received rewards for.

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"They were surprising, the crows had to keep the design of the tool in their mind while recreating it," Taylor said.

"We had no idea if the crows could do this."

Taylor said the results not only showed crows used their intelligence when using tools, but provided evidence that using mental templates may be another way to increase the complexity of tools culturally.

"So this is teaching us something about ourselves: mental template matching appears to be another route to cumulative cultural evolution."

The study followed another recent paper Taylor co-authored, which revealed how New Zealand's kea were also clever problem-solvers and often teamed up with others on tasks.