Clever detective work by an international team, including an Australian National University (ANU) researcher, has helped solve the mystery of which plants a population of crows in New Caledonia use to make tools.

The crafty crows are known for making their own hook-tipped pole tools for hauling invertebrate prey out of small holes and crevices.

The New Caledonian crow is the only non-human animal known to make hook tools in the wild.

“They put a lot of effort into making them. They use certain plants with forked stems that they remove and then turn into hook-shaped tools for foraging,” said study co-author Dr. Linda Neaves from ANU.

“But they remove leaves and much of the bark, making it impossible to quickly identify the plant species.”

While Dr. Neaves of the University of St Andrews in Scotland were able to determine which plants the crows used to make these tools at two of their long-term study sites, a third mystery remained puzzling.

“The plants used in one of the study areas are introduced shrubs, and in the other they appear to use a variety of raw materials. But no one had ever seen the crows making the tools at the third site, so we had no “idea what species it was,” said Matthew Steele of the University of St Andrews.

After years trying to figure it out using a number of approaches – including behavioral observations in the wild and in free range aviaries, radio location of birds, and working with local botanists to study collected tools – the crow team was at a loss.

Here comes Dr. Neaves into play.

“The field team was able to collect some tools and reached out to me and Professor Peter Hollingsworth of the Royal Botanic Garden in Edinburgh, Scotland, to ask if DNA could be used to find out which plant it was,” said Dr. Neaves.

She managed to match the DNA of the tools with a large native tree, the Spanish cherry or Mimusops elengi.

“Since it is native, this could be one of the original plants the crows used to make hook tools. The field team later confirmed that temporarily captive crows liked to make hook tools from this plant,” Mr. Steele explained.

“It was amazing to finally find out,” said Dr. Neaves.

“It was right in front of our eyes the whole time. When the crow researchers first explained this problem, it was hard to tell if we could give an answer. Usually you have a bit more to do, in this case it felt like looking for a needle in a haystack – there could have been a lot of plants in New Caledonia. ”

Dr. Neaves says it’s a reminder of all the little things that some species depend on in order to survive.

“Obviously these crows have adapted their remarkable tool making skills over time to take advantage of introduced plant species, and it’s exciting to discover that this population prefers an indigenous tree. It raises many interesting questions about how and why crows choose plants, “use them and how this can be affected by changes in their environment.”

Professor Hollingworth says the study “shows the broad utility of DNA barcoding for the essential task of separating and identifying plant species.”

The long-term study of crows was led by Professor Christian Rutz of the University of St Andrews, expeditions for this study were led by his graduate student Matthew Steele.

The study was published in PNAS.