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Where do birds’ legs go when they fly?

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Jack Johnsrud is 13, and most of the time I could swear he doesn’t know if it’s day or autumn, summer or night. That veil of oblivion is one of the characteristics of his age, but he’s more observant than I give him credit for.

We’re sitting on the tailgate of my pickup, watching a baseball game. A seagull flies over, and Jack cracks up. He thinks their squawk sounds funny. Then he studies a bird swooping over the outfield and asks a question that is deeper than he knows: “Where do their legs go when they fly?”

Jack has just struck one of the essential questions of anatomy, and the answer is fundamental to who we are, what birds are, and even helps answer a crucial evolutionary question.

It turns out that birds’ posterior anatomy is even more remarkable than their feathered wings. See, a bird’s legs are designed to move only at the knee. Humans, antelope, frogs, crickets — we all rotate our hips and trust our thighs when we walk or run. But birds have immovable thigh muscles. They walk by scissoring their knees, and you can imitate the motion by crouching down like a baseball catcher and crab-walking by moving just your lower legs.

This leg anatomy also explains why female birds can easily squat on their eggs. When we humans (especially those of a certain age) squat, we require plenty of room to thrust out our knees, then kneel, and then finally settle on our heels. Birds simply accordion their legs and settle straight down on eggs or nests.

Birds’ inability to move their hips and thighs means they have little trouble retracting their landing gear when they take flight. Species like seagulls and falcons simply tuck their thighs against their bellies and let their feet and talons hang. Birds like herons and cranes sweep their legs straight back.

This mechanism has evolutionary biologists suggesting that there’s no way that birds could have descended from dinosaurs, which have leg structures more similar to those of mammals.

I’m not smart enough to know whether that’s true, but I felt obligated to help Jack answer his question. One reason that birds can hold their legs up when they fly, I told him, is that avian bones are hollow and light compared to our heavy, dense bones. Jack nodded. Then he asked if we could crab-walk over to the concession shed for a snack.



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