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Tool-wielding crows are left- or right-beaked: Here's why

The super-smart New Caledonian crow tends to tip its head to one particular side when it is wielding tools. Researchers have figured out why.

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Crow image by Tambako the Jaguar, CC BY-ND 2.0

Crows can be a lot like us. They can use tools, they can figure out complex puzzles and they predominantly favour one side over another when using tools.

Although we don't really understand how handedness happens, we now know why New Caledonian crows -- famous for their use of tools, such as using sticks to extract grubs from burrows -- are left- or right-beaked. According to a team of researchers at the University of Oxford, it's because one eye is usually stronger than the other.

"If you were holding a brush in your mouth and one of your eyes [was] better than the other at brush length, you would hold the brush so that its tip fell in view of the better eye," said co-author Alejandro Kacelnik. "This is what the crows do."

The way the birds tilt their heads allows them to keep the tip of the stick in view of the eye on the opposite side of their head, the researchers said. Also, their study suggested that their extreme binocular vision -- that is, a very wide field of view compared to other species -- is optimised for viewing with one eye at a time.

"Binocular vision is often connected to allowing the brain to compare the images seen by each eye, inferring properties of the scene from the differences between these images," said study leader Antone Martinho. "We thought that their binocular fields would facilitate binocular vision, perhaps allowing the birds to judge the distance from tool tip to target. It turned out that, most frequently, they only see the tool tip and target with one eye at a time."

What this suggests is that the crows' wide field of view didn't develop so much for binocular vision as it did to allow a wider field of view for monocular vision; that is, to allow each eye to be able to see what is on the other side of the crow's beak.

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Mango showing a right bias using a stick (left) and Liane using her right eye to peer (right). Current Biology, Martinho III et al.

To arrive at this conclusion, the team performed an eye dominance test on nine crows to determine which eye they favoured. Four of the crows were right biased, while five were left biased. They then studied the crows using tools; using the tools, three crows were found to be right biased, while six were left biased. Although it was not a perfect predictor, eye dominance turned out to be a pretty reliable predictor for which side the crow preferred to use tools.

This is in contrast to humans, whose handedness, as far as we know, has no causative link to eye dominance.

"This contrasts with humans species-wide motor laterality and uncorrelated motor-visual laterality, possibly because bill-held tools are viewed monocularly and move in concert with eyes, whereas hand-held tools are visible to both eyes and allow independent combinations of eye preference and handedness," the study reads. "This difference may affect other models of coordination between vision and mechanical control, not necessarily involving tools."

But that doesn't mean we need to say goodbye to kinship with our feathered friends: "Birds and humans face similar problems in tool use and many other activities," Kacelnik said. "Studying similar problems across species helps to put all of them in perspective."

Plus, crows rock some sweet shreds.

The full study, "Monocular tool control, eye dominance, and laterality in New Caledonian crows", can be found online in the journal Current Biology.