New Caledonian crows (Corvus moneduloides) pass stones to get food, but don't seem to realize they need a partner. University of Auckland

The crows of New Caledonia are possibly the most extraordinary tool users apart from humanity. Yet, among their remarkable talents, a weakness has been found. For all their capacity to use stonestwigs and wire, it seems these masterminds of the bird world have not really grasped the concept of cooperation.

Many species of the corvid family show unusual intelligence, but when it comes to tool use, the crows of New Caledonia are the masters. Don't believe us? Ask yourself how many humans you know could pass this test as quickly as the crow did. 

Not long ago, Ph.D. student Sarah Jelbert of the University of Auckland revealed that crows understand the way solid objects displace water about as well as children aged 5-7.

Now, in a new paper published in PloS One, Jelbert finds that crows 'collaborating' to get food don't seem to understand what they are doing, particularly that teamwork requires a teammate.

For the study, Jelbert placed pairs of crows in a subdivided cage. To access treats, the crows needed to drop stones into a trap. One crow had the stone and the other had access to the hole the stone needed to be dropped into to gain food. The only route to the food was for one crow to pass the stone to the other, who would put it in the hole, enabling both to dine.

 

 

The crows quickly learned to perform the operation, apparently providing further evidence of their intelligence. That is, until Jelbert tried to vary the conditions, including removing the second crow from the operation. “The crows did not adjust their behavior according to the presence or absence of a partner,” Jelbert reports. None of the six birds used in the trial were more likely to pass a stone when there was another crow there to receive it than when there was not.

With that discovery, it was also unsurprising that the crows did not show other signs associated with cooperation, such as inequity aversion, where animals such as monkeys refuse to cooperate when their partner gets an unfair slice of the reward.

The findings are somewhat surprising since New Caledonian crows, while not the most social of birds, have been known to cooperate to mob raptors intent on murdering the murder (the term for a group of crows). 

Jelbert told IFLScience that more tests are required to uncover why crows that can work out how to use one tool to get another apparently can't understand that “another crow can also be a tool.” Possible theories include that they have “domain specific cognition, not general cognition like us.” On the other hand, she acknowledged, “Maybe they had got so used to dropping stones they just did it automatically.” She also acknowledges that a redesigned cage, where the hole for passing stones is not so close to where they need to be dropped, might influence the result.

Whatever the answer, Jelbert thinks it interesting that, “complex social cognition does not appear to be a prerequisite for… producing complex tool behaviors and making causal inferences.”

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