Source: Photo: Natalie Uomini via Flickr. Distributed under a CC BY-NC-SA 2.0 license.

New Caledonian crows can guess the weight of an object by watching how it behaves in the wind, according to an international team of researchers. The findings, published in Proceedings of the Royal Society B, are the first of their kind in a non-human animal.

Humans use a variety of cues to gauge an object’s weight without directly handling it, including how easily it can be moved. For instance, if we see one object blowing down the street in the wind while another object remains stationary, we can infer which object is heavier.

To find out if New Caledonian crows also make judgments about weight through observation alone, researchers first trained twelve wild-caught crows to discriminate between light and heavy objects. Half of the birds were rewarded when they dropped light objects into a food dispenser and half were rewarded for choosing heavy objects.

Next, the birds observed pairs of novel objects suspended from strings in front of an electric fan. When the fan was on, the light objects blew around easily, while the heavy ones remained stationary. When the fan was off, both objects remained still.

Finally, the birds were allowed to pick one of the objects to drop into their food dispenser. Despite having no opportunity to handle the objects prior to testing, crows who observed the objects buffeted by the fan were 73 percent correct in choosing the correct object (the one that would get them rewarded). If the test was performed with the fan switched off, the birds did no better than chance.

You may have heard of New Caledonian crows before – they are famously fast learners that make and use tools regularly in the wild and captivity. Determining the weight of objects may be ecologically relevant to New Caledonian crows, as they drop both candlenuts and snails from heights onto hard surfaces to break open and then eat. The weight of a nut can indicate whether it is good to eat or rotten and snails can be heavy relative to a crow’s body weight.

Other animals have not been tested on this particular task, so it is not clear if this ability is special or widespread throughout the animal kingdom. Previously, some researchers claimed that only humans have a full, generalizable concept of weight. This study shows that New Caledonian crows, at least, are capable of making inferences about the weight of objects without direct contact with objects.