A clever piece of detective work has helped solve the mystery of how a crafty population of Pacific island crows is making tools.
New Caledonian crows are well known for crafting stick tools with hooked tips to extract small prey from holes and crevices.
They are the only non-human animal known to manufacture hooked tools in the wild, Australian National University researchers say.
"They put a lot of effort into making them," evolutionary biologist Linda Neaves said.
"They use specific plants with forked stems, which they remove and then process into hooked tools for foraging."
The crows also remove the leaves and bark, making it often impossible to identify all the plant species they're using.
After years of trying to work it out using a range of scientific approaches, including behavioural observations in the wild, radio-tracking and working with local botanists to examine collected tools, researchers were at a loss.
That is until ANU's Dr Neaves used DNA evidence to work out that the mysterious plant was a large native tree called the Spanish Cherry.
The field team later confirmed that temporarily captive crows happily crafted hooked stick tools from the same plant.
"It was amazing to finally work it out," Dr Neaves said.
"It felt like searching for a needle in a haystack. It could've been a large number of plants on New Caledonia."
The long-term New Caledonia crow study was led by Professor Christian Rutz from the University of St Andrews, with expeditions for this study led by his PhD candidate, Matthew Steele.
The study has been published in the peer-reviewed Proceedings of the National Academy of Sciences of the United States of America (PNAS).
New Caledonia is a special collectivity of France in the southwest Pacific Ocean, about 1210 km east of Australia.
Australian Associated Press