Crows look after their valuable tools more carefully

New Caledonian crows are unusual among birds in that they use tools to extricate food items from holes in the stems of trees or from cracks in the bark. The crows forage in this way using two different types of stick tools – a straight twig or leaf stalk that can be collected from the ground, or a hooked stick that is fashioned specifically by the crow from stems of a particular species of plant. 

When crows forage using a hooked stick tool, they are up to ten times more efficient at securing food. This indicates that the stick tools are an important part of the foraging strategy and clearly help crows to access food successfully. The awkward part is that a crow cannot hold its stick tool in its beak while at the same time eating a food item it has just extracted; while eating, the bird has to place its tool somewhere other than in its beak.

In previous research on NC crows, scientists from the School of Biology at the University of St. Andrews in Scotland found that crows secure their tools using one of two “safekeeping” strategies while they eat their food – they either hold the tool trapped underfoot, or they temporarily insert the tool into a nearby hole or behind bark. If the tool is not securely stored there is a risk that it falls to the ground and has to be retrieved, or that another crow steals the tool for its own use.

Researchers from the University of St Andrews and the Max Planck Institute of Animal Behavior in Konstanz, Germany, have now investigated whether NC crows take better care of their favorite tools by storing them more securely when not in use. Their results were published recently in the journal eLife.

The study involved two main experiments using a large sample of wild-caught, temporarily captive New Caledonian crows that were presented with food inserted into holes drilled in a piece of wooden log. In the first experiment they investigated whether crows prefer one tool type over the other, and in the second they assessed whether crows take better care of their preferred tools by storing them carefully between periods of use. 

When offered a choice of tools, either hooked sticks or straight sticks, the crows clearly preferred the hooked sticks. While straight sticks may be picked up commonly from the leaf litter below the trees, hooked stick tools have to be fashioned by the crows and demand time and energy investment. In addition, the crows select the stems of a particular plant, Desmanthus virgatus, from which to craft the hooked tools. Since this plant is not very common, wild crows have to invest energy in searching for these specific plants as well. 

“Hooked tools are not only more costly to obtain, but they are also much more efficient,” explained team leader, Christian Rutz from the University of St Andrews. “Depending on the foraging task, crows can extract prey with these tools up to ten times faster than with bog-standard non-hooked tools.”

Assuming that the crows would ‘value’ the hooked tools because of their effectiveness and the investment required to produce them, the researchers then used video footage to monitor how crows stored their tools after use. 

“Many of us will fuss about a brand-new phone, making sure it does not get scratched, dropped or lost. But we may handle an old phone with a cracked screen quite carelessly,” said lead author Barbara Klump, who is now based at the Max Planck Institute of Animal Behavior, Germany.

The videos were analyzed to determine whether crows stored their tools securely (trapped under a foot or placed in a hole), or whether they left them lying around on top of the log or on the ground. The results showed that the crows were significantly more likely to keep their tools safe after foraging with a hooked stick. Furthermore, they used the safest option (a hole) to store a hooked stick more frequently than to store a straight stick. 

“It was exciting to see that crows are just that bit more careful with tools that are more efficient and more costly to replace. This suggests that they have some conception of the relative ‘value’ of different tool types,” noted study co-author James St Clair.

This is the first study to investigate how animals handle and store tools of different kinds, providing an innovative way to measure how much they value these objects. The method can be applied using untrained subjects and it allows for behavioral comparisons between tool-using animals from different taxonomic groups. 

Image Credit: James St Clair

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By Alison Bosman, Earth.com Staff Writer