A University of St Andrews-led study shows that New Caledonian crows (Corvus moneduloides) acquire hidden food several times faster when using hooked rather than basic (non-hooked) tools, regardless of tool material, prey type and extraction context.
The New Caledonian crow (Corvus moneduloides) is the only non-human animal known to craft hooked tools in the wild. Image credit: James St Clair.
New Caledonian crows — a species of oscine passerine bird found on the South Pacific island of New Caledonia — are famous for their use of tools to winkle beetle grubs and other small prey out of hiding places.
Although they are capable of extracting food with straight twigs, in some areas they actively manufacture hooked stick tools before going hunting.
“It’s a painstaking sequence of behaviors,” said study lead author Dr. James St Clair, from the School of Biology at the University of St Andrews.
“New Caledonian crows seek out particular plant species, harvest a forked twig, and then — firmly holding it underfoot — carve, nibble and peel its tip, until it has a neat little hook.”
Researchers have long assumed that there was some benefit to crows manufacturing hooked tools, but had no idea just how much better they might be.
Dr. St Clair and co-authors conducted experiments to record how long wild-caught crows took to extract food from a range of naturalistic tasks, using either hooked or non-hooked tool designs.
Depending on the task, they found that hooked tools were 2-10 times more efficient than non-hooked tools.
“That’s a huge difference. Our results highlight that even relatively small changes to tool designs can significantly boost foraging performance,” said study senior author Professor Christian Rutz, also from the School of Biology at the University of St Andrews.
“These new findings help explain why New Caledonian crows have evolved such remarkable tool-making abilities,” added co-author Professor Nick Colegrave, from the School of Biological Sciences at the University of Edinburgh.
“In nature, getting food quickly means that birds have more time and energy for reproduction and steering clear of predators. It’s really exciting that we were able to measure the benefits of these nifty crow tools.”
Scientists still don’t know how crows acquire the ‘know-how’ and make hooks; they may inherit the ability from their parents, or learn by observing experienced birds.
Either way, because hooked-tool users will live longer and leave more offspring, the skill is expected to spread.
“We’ve all heard that the early bird gets the worm. In the case of the New Caledonian crow, it’s the skilled hook-maker that gets the worm, or at least it gets many more worms than its less-crafty neighbors,” Professor Rutz said.
The study appears today in the journal Nature Ecology & Evolution.
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James J. H. St Clair et al. Hook innovation boosts foraging efficiency in tool-using crows. Nature Ecology & Evolution, published online January 22, 2018; doi: 10.1038/s41559-017-0429-7
