Crows Can Judge An Object’s Weight By How It Moves – Something Only Seen Before In Humans


Rachel Baxter

Copy Editor & Staff Writer

New Caledonian crows are deceptively smart. Sarah Jelbert

While folklore portrays the owl as the wisest of birds, another feathery critter certainly takes the title. Crows build their own tools, take on larger rivals in sneaky coordinated teams, and understand certain concepts better than human children. They’ve even been spotted buying train tickets

Adding to this list, researchers at the University of Cambridge have discovered that these cunning creatures can work out the weight of an object just by observing how it moves in a breeze. This kind of inference has previously only been observed in humans. 


If we look at a cardboard box, we can guess how heavy it is based on how it’s affected by the wind. An empty box will blow away down the street, while one filled with books will stick to its position. 

So how do you test such a complex concept on animals?

The researchers gathered a gang of 12 New Caledonian crows and trained them to differentiate between two objects – one heavy and one light. Half of the birds were trained to drop the heavy object into a plastic tube, receiving a treat if they did so, while the other half learned to go for the lighter object.

Then, the team dangled two objects, one heavy and one light, from strings. These objects were unfamiliar to the birds, and placed in front of a fan. When the fan was switched on, the birds observed how the objects moved in the breeze – those trained to pick up heavy objects selected the heavier one, and vice versa. This didn’t happen when the fan was switched off, suggesting that the crows were working out which object was heavier based on how it blew in the wind. The findings are published in the Proceedings of the Royal Society B

A crow weighing up which of two objects is heavier by watching them move in the breeze. Sarah Jelbert

“This particular ability has never been tested before, so we had no idea how the crows would behave,” lead author Dr Sarah Jelbert told IFLScience. “That they passed so conclusively was a really exciting finding! It demonstrates that drawing these types of conclusions about how the world works comes very naturally to these birds.”

Being able to gauge weight is a useful skill in the wild. Jelbert explained to New Scientist that crows drop nuts on the ground to crack them open and their weight indicates whether they’re rotten or ready to eat.  

So, might other animals use the same clever tactic to assess the weight of objects in their environment? “I'd love to find out!” Jelbert told IFLScience. “We just don't know at this stage, but it would be wonderful to see this type of experiment conducted with parrots, primates, as well as some of the species that we don't usually think of as being intelligent. It might be that it's actually very widespread, but we have no idea just yet.” 

Janis the crow taking part in the experiment. Sarah Jelbert


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