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Clever crows may grasp hidden causes

Crows may be imagining more than we imagined. New research suggests certain crows make decisions based on factors they can’t see. A recent study published in the Proceedings of the National Academy of Sciences (PNAS) deepens our understanding of these crafty corvids, and could help explain how human reasoning evolved.

Crows are intelligent problem solvers, capable of making hook-shaped tools to retrieve food and using multiple tools in a logical sequence. New Caledonian crows (Corvus moneduloides) are particularly adept tool users, and have often been the subject of cognitive research. In this study, the New Zealand–based researchers tested whether New Caledonian crows could trace an event back to a cause that was hidden from their view.

A New Caledonian crow using a tool to retrieve food
A New Caledonian crow using a tool to retrieve food. Photo by Alex H. Taylor.

The crows first learned to extract meat stashed inside a box using a tool. Then the scientists placed the box near a large hanging sheet with a hole in it. In one scenario, a crow watched a human walk behind the sheet, saw a stick emerge threateningly from the hole and disappear, then saw the human leave. To extract food, the crow would have to place its head within poking distance of the hole and face away from the sheet. Despite the risk, crows showed little hesitation while wielding their tools. They apparently attributed the stick’s movement to the hidden human, and perceived no danger once the human left.

In a second scenario, the team moved the stick remotely, without a person behind the sheet. All eight crows in the study approached the food-filled box more cautiously. If no one had left the hiding place, the birds apparently reasoned, the stick might appear again and poke them in the head. They stopped feeding more often to inspect the sheet. Four crows eventually dropped their tools and left, deeming the whole operation too risky.

Evolutionary psychologist Alex Taylor discusses his team’s latest experiment with New Caledonian crows, which suggests they use human-like reasoning at times.

“We didn’t really expect the crows to make this distinction,” evolutionary psychologist and lead author Alex Taylor of the University of Auckland told He predicted the test would “show a big difference between humans and animals.” Instead, it suggests that crows can understand hidden causes—an ability that underpins scientific thought in humans, the researchers say.

Using memory to infer an unseen threat has never been demonstrated in nonhumans before, said John Marzluff, a wildlife biologist at the University of Washington, in an interview with “Things that are out of sight are not out of their mind,” said Marzluff, who was not involved in the study.

Other scientists suggest these tests of animal reasoning merely reflect conditioned behavior. For example, a tropical bird often sees leaves rustling in the forest canopy, and then gets a glimpse of a monkey. The bird might be conditioned to associate rustling with monkey, but it’s not necessarily reasoning about a hidden monkey.

Taylor argues that this explanation doesn’t account for his results. Since the crows confidently placed their heads into the feeding area after the very first human trial, he said, they must have reasoned that the danger of being poked had vanished when the person left. They weren’t categorically associating stick or human with danger; rather, they made two different assessments of risk in the two scenarios, Taylor maintains.

The researchers plan to test whether parrots—highly social birds that don’t use tools—can reason about hidden causes, too. They hope to learn whether tool users or social species face complex problems that require causal reasoning, and whether this effect drove the evolution of such reasoning in Homo sapiens.

CITATION: Alex H. Taylor, Rachael Miller, and Russell D. Gray. New Caledonian crows reason about hidden causal agents. Proceedings of the National Academy of Sciences, 17 September 2012. DOI: 10.1073/pnas.1208724109

Kelly Servick is a graduate student in the Science Communication Program at the University of California, Santa Cruz.

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