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Crows 'clever' as human seven-year-old

Clever crows A species of crow can solve a problem based on an Aesop's fable as well as an average seven-year-old, a new study shows.

Like their counterparts in Aesop's tale of the Crow and the Pitcher, New Caledonian crows 'understand' that dropping rocks into a partially filled tube of water will raise its level high enough to access a reward.

However a paper published today in PLOS One shows this knowledge has its limits and gives an insight into the evolution of an understanding of cause and effect.

New Caledonian crows, Corvus moneduloides, native to the archipelago with which they share their name, have "exceptional tool manufacturing abilities, routinely making and using tools in the wild as well as in captivity", the researchers write.

However Sarah Jelbert from the University of Auckland, who led the latest study, wanted to test the boundaries of crow knowledge.

"These [previous] studies tend to show how well the birds perform certain tasks," she says.

"But fewer look at when they fail, that is, explore the limits of their understanding of cause and effect relationships."

Aesop's fable

To test whether the crows understand the cause of water displacement the researchers used an example from Aesop's fables.

In Aesop's tale a crow dying of thirst comes across a jug of water whose contents are out of reach of its beak. After failing to push it over, the bird drops in pebbles one by one until the water rises to the top of the jug, allowing it to drink.

The tale is credited as the inspiration for the proverb, necessity is the mother of invention and where there's a will, there's a way.

To replicate the tale, six, wild-caught crows were given a week's basic training, which taught them dropping stones into a tube would produce a reward.

They were then given six tasks to complete. Over the course of the experiments the researchers found the crows preferred to drop stones into a water-filled tube instead of a sand-filled tube, Jelbert says.

They also learned to drop heavy objects that sank and displaced more water rather than floating objects, and chose solid objects rather than hollow objects.

"[This] is a difficult concept to understand and showed they are sensitive to quite subtle differences in their tools," says Jelbert.

The crows also opted to drop objects into a tube with a high water level rather than a low one.

"The results show the crows possess a sophisticated understanding of the causal properties of volume displacement, similar to that of five- to seven-year-old children," Jelbert adds.

Pushing the boundaries

However despite their impressive early achievements, when it came to choosing between wide and narrow tubes, and understanding the relative effort involved in raising water levels in the different tubes, things fell apart.

They showed no preference for either tube, and simply didn't recognise that the narrow tube needed only a few stones to raise the level compared with the wide tube.

One of the six crows was so demoralised by its lack of success after depositing all 12 of its stones into the wide tube for no reward, it wouldn't participate in any further experiments.

"It couldn't solve the problem, and figured it was not worthwhile to try so hard for no reward," Jelbert says.

The final task required them to understand a counterintuitive problem.

The crows were offered a U-shaped apparatus partially filled with water, with the tube's connection hidden under a platform; the part of the U-tube containing the reward was also too narrow for stones to be dropped in.

A second straight tube was positioned adjacent to the tube containing the reward.

To get the reward the crows had to decide to drop stones in the wider-mouthed segment of the U-tube, displacing the water in both that tube and the one containing the reward. All of the crows failed to complete the task successfully.

"These results are striking as they highlight both the strengths and limits of the crows' understanding," Jelbert says.

"In particular, the crows all failed a task which violated normal causal rules, but they could pass the other tasks, which suggests they were using some level of causal understanding when they were successful.

"While there is more work to be done, this research is helping to build our understanding of animals' cognition and how intelligence has evolved in animal species other than humans."