Media playback is unsupported on your device Media caption Dr Christian Rutz described his realisation that the crows might use tools as a 'eureka moment',

A bird so rare that it is now extinct in the wild has joined a clever animal elite - the Hawaiian crow naturally uses tools to reach food.

The bird now joins just one other corvid - the New Caledonian crow - in this exclusive evolutionary niche.

Dr Christian Rutz from St Andrews University described his realisation that the bird might be an undiscovered tool user as a "eureka moment".

He and his team published their findings in the journal Nature.

"I've been studying New Caledonian crows for over 10 years now," Dr Rutz told BBC News. "There are more than 40 species of crows and ravens around the world and many of them are poorly studied.

"So I wondered if there were hitherto undiscovered tool users among them."

Previously, Dr Rutz and his colleagues have reported that New Caledonian crows have particular physical features - very straight bills and forward-facing eyes. The researchers suggested these might be tool-using adaptations.

Breeding programme

They then searched the crow family for species with similar features, and Dr Rutz said he quickly realised that the "Hawaiian crow was the perfect candidate for further investigation".

Though it will now be something of a scientific celebrity, the Hawaiian crow has recently been rescued from the very brink of extinction.

Dr Rutz worked with colleagues at San Diego Zoo Global, who had brought the last remaining wild birds into captivity to start a breeding programme to save the species - those birds provided the scientist with a unique testing ground.

"We effectively tested the entire species," Dr Rutz told BBC News.

"At the time, there were 109 crows in captivity - we tested all of them, presenting them with a foraging task."

Crow in crisis

The Hawaiian crow or 'alalā (Corvus hawaiiensis) is extinct in the wild and all the birds that remain are being kept in captive breeding. The last few birds were brought into captivity in a desperate effort to save this species from extinction. There are now 130 birds in captivity, and the team at San Diego Zoo Global are preparing for them to be released into the wild later this year.

According to zoo president Douglas Myers, this study marks an important milestone for the recovery programme.

"The discovery that 'alalā naturally use tools is of great significance," he said, "especially at this critical stage of our recovery efforts, as it provides completely unexpected insights into the species' ecological needs."

That task consisted of logs with holes and crevices that were baited with food that was just out of a bill's reach.

"They were able to pick up sticks from the aviary," said Dr Rutz, "and of all the birds we tested, 93% used [the sticks as] tools. This suggests this is a species-wide skill.

"They were incredibly dextrous in the way that they handled the sticks, shortened them when they were too long, and discarded them if they were not happy with them."

Image copyright Lisa Bissi Image caption The birds found and used sticks from their aviary

Since New Caledonian and Hawaiian crows both evolved on remote islands, the researchers think "something special" is happening on these islands that, at least in birds, drives the evolution of this highly unusual behaviour.

Dr Rutz added that the discovery also opened up the possibility of comparing these tool-using birds with primates that have similar skills, to work out what drives the evolution of these skills on such very different branches of the evolutionary tree.

Well-known primatologist Dr Jane Goodall provided the first detailed report of tool use in wild chimpanzees and a St Andrews University statement quoted Dr Goodall as describing this finding as "especially wonderful".

"Each of these discoveries shows how much there is still to learn about animal behaviour, and it makes me rethink about the evolution of tool use in our own earliest ancestors," she commented.

"Let this discovery serve to emphasise the importance to conserving these and other animal species so that we can continue to learn ever more about the range of their behaviour before they vanish for ever in the sixth great wave of extinction. We owe it to future generations."

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