Why Birds Can Sleep on Branches and Not Fall Off

Their talons work through a series of pulleys made of tendons, as these GIFs and drawing explain.
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Ars Anatomica

You ever see a bird clutching onto a branch high in a tree and wonder, "What happens if it falls asleep? How could it hold on?" 

The avian talon works through a "pulley system of tendons," according to the animal morphology blog Ars Anatomica, and it can lock into place. 

"The bird's foot closes and grasps automatically as the ankle and knee joints are bent," we read. "This grasp cannot be released until the limb is straightened again." 

So, instead of expending precious energy holding the muscles tight—as you would if you were hanging onto a branch with your fists/arms—the system simply physically locks in place. 

A 1990 paper in Zoomorphology goes into more depth about the biomechanics of the lock, which results from the placement of the tendons and evolved specializations of their texture. 

And, of course, this wondrous anatomical system doesn't just come in handy on telephone wires, but also during attacks:

Raptors swoop down on prey with talons/legs outstretched. The impact with the prey folds the raptor’s legs against its body, causing the talons to clench automatically, tearing into the prey. The automatic grip is strong enough to kill, and is what allows many hawk species to catch and kill other birds in midair. 

UPDATE: It appears that this mechanism — as described in the paper and post above — is an area of active scientific dispute (!). A 2010 paper in the Journal of Experimental Zoology Part A found that such a mechanism did not exist in the European starling. Thanks for the update, Ed Yong.

Ars Anatomica

Via Deb Chachra

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Alexis C. Madrigal

Alexis Madrigal is a senior editor at The Atlantic, where he oversees the Technology Channel. He's the author of Powering the Dream: The History and Promise of Green Technology. More

The New York Observer calls Madrigal "for all intents and purposes, the perfect modern reporter." He co-founded Longshot magazine, a high-speed media experiment that garnered attention from The New York Times, The Wall Street Journal, and the BBC. While at Wired.com, he built Wired Science into one of the most popular blogs in the world. The site was nominated for best magazine blog by the MPA and best science Web site in the 2009 Webby Awards. He also co-founded Haiti ReWired, a groundbreaking community dedicated to the discussion of technology, infrastructure, and the future of Haiti.

He's spoken at Stanford, CalTech, Berkeley, SXSW, E3, and the National Renewable Energy Laboratory, and his writing was anthologized in Best Technology Writing 2010 (Yale University Press).

Madrigal is a visiting scholar at the University of California at Berkeley's Office for the History of Science and Technology. Born in Mexico City, he grew up in the exurbs north of Portland, Oregon, and now lives in Oakland.

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