New Caledonian Crows Estimate an Object's Weight in an Ingenious Way

Crows are special. They make tools from memory, and they even gather around their dead. And the list of what makes them special gets longer every day. Research published on Wednesday in the journal Proceedings of the Royal Society B suggests that New Caledonian crows, a species from the South Pacific, can infer the weight of an object just by watching it blow in the wind. This finding makes New Caledonian crows the only known animal species — besides humans — that can make these judgments without physically interacting with the objects.

In the new paper, a team of researchers outlined a series of experiments in which they found that crows could tell, just by watching two boxes get blown around by an electric fan, which box contained the heavier object. This type of inference may seem logical to humans — a candy bar caught in an updraft, for instance, is obviously lightweight, while a coconut falling from a tree is clearly heavy. Among non-human animals, though, this type of perceptive ability is actually quite rare. Even chimpanzees, who share 98.8 percent of their DNA with humans, have been unable to infer the weight of objects in experiments. But Sarah Jelbert, Ph.D., a post-doctoral research associate in the Department of Psychology at the University of Cambridge and the paper’s first author, says the ability comes quite naturally to crows.

“Recognizing that objects are heavy or light from seeing them blowing in the wind appears to be very intuitive to these birds,” Jelbert tells Inverse.

Jelbert and her collaborators trained 12 wild-caught New Caledonian crows to drop objects into a Plexiglas tube to receive a reward (a small piece of meat on a bottle cap). After they’d mastered this basic task, they moved onto a slightly more complex one: Six of the birds learned to drop only light objects into the tube, while the other six birds did the same with only heavy objects.

Once the birds consistently selected only light or only heavy objects, the scientists created two different conditions for the “experiment proper”: the experimental condition, in which two objects suspended by string were blown by an electric fan, and a control condition, in which the objects were still suspended from strings but rested on the table with the fan turned off. Birds were put through both conditions each day, in which they observed the objects and then had to drop the right one in the tube, depending on how they’d been trained. The crows grasped the task quickly, correctly selecting the heavier box 73 percent of the time in the experimental trials.

Scientists know that crows are good at learning — sometimes scarily so — but this phenomenon shows more than that, argue Jelbert and her team. They think that “mental weighing” actually serves an important survival function in the wild. In previous research, corvids (the family crows belong to) select nuts by weight, presumably to determine whether a nut is full before opening it. And so, it’s thought that the ability to guess at a nut’s weight by watching it sway in the wind while it’s still on a tree branch is a helpful skill in the wild.

Jelbert suggests there’s yet another use for the ability that also involves foraging but is a little less obvious.

"What we need is to run this test with other species to find out.

“Understanding risks is a big one,” she says. “If you can tell the difference between something heavy (and dangerous) and light (and therefore not very harmful), you can make better decisions about whether a given situation is safe.” For example, it’s in the crow’s best interest if it can determine whether it’s safe to spend time with a certain object hanging overhead.

While the team’s results show that New Caledonian crows can learn this new skill quickly and accurately, Jelbert says she suspects they’re not the only animals that can do it. As such, future research should focus on whether other animals can infer the weight of objects without actually touching them.

“We know that these crows are particularly impressive, they manufacture tools in the wild and learn new rules exceptionally quickly,” she says. “This ability may be unique to these birds, but I have a suspicion that it may be much more widespread. What we need is to run this test with other species to find out.”