Carrion crows are turning cars into makeshift nutcrackers, placing walnuts in their path at traffic lights and returning for the cracked food: What Japanese researchers found in a new study

Carrion crows have been observed turning busy roads into makeshift nutcrackers, placing walnuts in the path of vehicles so passing cars can break the hard shells. Once the vehicle moves away, the birds return to pick up and eat the exposed pieces.

The behaviour was documented repeatedly rather than being based on a single observation. Research published in the Japanese Journal of Ornithology recorded 43 instances over a 21-month observation period. The study, titled Variations of behaviour of Carrion Crows Corvus corone using automobiles as nutcrackers, examined how the birds placed walnuts, waited for vehicles and retrieved the cracked pieces.

The researchers found several variations in the behaviour, including where the walnuts were positioned, how long the crows waited and whether they moved the nuts before returning to them. The findings suggested that the birds were systematically using vehicles to solve the problem of opening walnuts that were difficult to crack.

How carrion crows used cars to crack walnuts

The process followed a relatively clear sequence. A crow would take a walnut and place it on the road, often in front of a vehicle stopped at a traffic signal. When the vehicle moved forward, its tyres could crush the walnut and break the shell.

After the car passed, the crow would return to the road and feed on the broken pieces.

Yoshiaki Nihei documented the behaviour over 21 months, recording 43 occurrences in detail. However, the crows did not follow exactly the same routine every time.

Their behaviour differed depending on factors such as how they obtained the walnut, its condition, where they placed it, the time of day and whether other crows were nearby. The birds also varied how long they waited for vehicles and whether they repositioned walnuts before retrieving them.

Why researchers considered the behaviour systematic

The repeated observations indicated that the crows were not simply leaving walnuts on roads by chance. Researchers examined several aspects of the process, including how the birds positioned the nuts, how they waited for vehicles and whether they adjusted a walnut's position.

The presence of other crows also appeared to influence some of their behaviour. The researchers observed differences in how the birds retrieved and consumed the cracked walnut pieces.

Rather than finding that every crow behaved in exactly the same way, the study identified a recurring sequence with several flexible elements. Based on these observations, Nihei considered the use of automobiles a systematic and intentional method of obtaining walnut fragments.

Crows also use height to crack walnuts

Using cars was not the only method available to the birds. The study also described carrion crows dropping walnuts from heights of more than 10 metres in an attempt to break their shells.

However, some walnuts were too hard to crack using this method. Nihei suggested that automobiles provided an alternative for walnuts that were particularly difficult to open or were still covered with pulp.

This suggests that the crows can use more than one strategy when dealing with hard-shelled food. Dropping walnuts from a height and placing them in the path of vehicles represent different approaches to the same problem.

What the 21-month observations revealed

The significance of the observations lies not only in the crows' use of vehicles but also in their flexibility. Over the 21-month study period, the birds varied where they placed walnuts, whether they repositioned them, how long they waited and how they retrieved the broken pieces.

The repeated behaviour and variations in the sequence led the researcher to conclude that the car-using method was systematic and intentional.

The observations provide an example of how wild birds can adapt their feeding strategies to human-made environments, using existing infrastructure to overcome a problem that would otherwise require considerable effort.