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IPR News Radio's Sunday host, Cheryl Bartz, tells us what to look for as we wander around northern Michigan, helping us notice the little wonders all around us.

The bird with the skinny legs

How do geese tolerate cold temperatures on their bare feet? They have several strategies, including a down coat and few cold receptors in their feet. (Photo Credit: Paul F. Kisak)
Paul F. Kisak via Wikimedia Commons
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How do geese tolerate cold temperatures on their bare feet? They have several strategies, including a down coat and few cold receptors in their feet. (Photo Credit: Paul F. Kisak)

Geese have lots of ways to keep their feet warm. For starters, they can balance on one foot and draw the other one up into their downy breast. There’s a reason down jackets are highly sought after by humans. The little feathers trap air and dead air is an excellent insulator.

The goose (or any bird, really) can alternate feet. If that doesn’t warm them sufficiently, they can crouch and envelop both feet in down.

But sometimes they have to move around and that’s when counter-current heat exchange shines.

The arteries and veins in bird legs are closely intertwined. Blood leaving the heart has been warmed up—it’s in that down covered body! The warm blood warms the feet. But—here’s the special magic—the warm blood in the arteries passes near the cold blood returning from the feet and the warm blood transfers some heat to the cold blood. That’s counter-current heat exchange.

The arterial blood is cooled a little as a result, so it loses less heat when the foot touches snow or cold water.

And the venous blood is a little warmer, so it requires less energy to warm it when it gets back to the heart.

The feet are mostly sinew and scaly skin and have fewer cold receptors.

You might have noticed that most birds don’t have bulging thigh muscles–unless they were bred for your dinner table. The muscles in wild birds are mostly located high up on the leg, near that warm, downy body.

And that is why birds have warm-enough feet and skinny legs.


Honking goose audio (XC307648) from www.xeno-canto.org recorded by J.R. Rigby.

Cheryl Bartz hosts IPR's Sunday programming and writes a (mostly) weekly essay called "What's Up Outside?"