February 10, 2024
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The songbirds in your backyard jump around with such tiny legs. Here’s why bird legs are so thin and how they can support the weight of a bird
A bird in flight is poetry; A bird on the ground presents an enigma. If you watch a sparrow or other songbird swaying and scratching on the forest floor, it’s easy to wonder: How do they support their weight on such thin little legs?
The first reason birds’ legs appear strangely thin is that the rest of their body is covered mostly by feathers, which can dramatically magnify their apparent size, says Julia Clarke, a paleontologist who studies the evolution of birds. birds at the University of Texas at Austin. The relatively fleshier thighs of birds are tucked under the feathers, and most birds have scaly undersides, Clarke says, making these limbs appear strangely small and bare. Birds that have feathers down to their toes (such as owls and some domestic chickens) appear to have thicker shins than their bare-footed cousins.
But it’s not just a visual illusion: birds’ feet have some real differences from the feet of other living animals. Birds descend from theropod dinosaurs, a group of bipedal animals of different sizes. Some were as small as modern sparrows, while others were huge, like Tyrannosaurus Rex. While most mammals that can walk bipedally (such as bears, chimpanzees, and humans) stood on their heels, dinosaurs walked on their toes and the front of the foot, with the rest of the bones of the foot elongated and kept away from the ground. Living birds maintained some of these characteristics. However, as they rose higher and higher into the air, the birds developed some divergent traits. Bird relatives like velociraptor They had relatively short thighs above the shins and toes, but they kept them more or less vertical, like ours. However, if we travel down the bird family tree toward modern families, those thighs gradually change orientation. “In birds, the upper leg bone is held largely horizontal and is closely attached to muscle groups that attach to the trunk of the body,” says Clarke.
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Why did this change occur? Maintaining balance as a biped is a tricky task, as anyone who watches a toddler walk can attest. Without a strong weight center supported by sturdy legs, any two-legged animal is at risk of falling. Bipedal dinosaurs stood by distributing their weight between long bony necks and tails. But as birds’ ancestors took flight higher and higher, Clarke says, the development of powerful chest flapping muscles in birds shifted their center of mass toward the front of the body. To maintain balance, the birds’ torsos shrank. His thighs moved horizontally and became attached to the body, effectively becoming part of the hip. The knee, in turn, became the highest moving part of the leg.
Birds also developed other anatomical tricks. Over time, the three metatarsal bones found in the legs of dinosaurs transformed into a single fused bone in birds that was both light and strong. The birds also developed muscle changes, Clarke says, and most of their leg muscles consolidated into the thigh, which is attached to the torso. Meanwhile, the lower legs and feet are operated by bundles of superfine tendons that run from the knees to the toes.
That same system of tendons locks birds’ toes in position as they perch, allowing them to sleep without falling. “No other animal has this system of tendons that allows them to localize the strong muscle action close to the body,” where it is hidden under feathers, Clarke says.
Of course, not all birds have stick-thin legs. Birds that spend a lot of time on the ground (or make a living grabbing prey) still have characteristically thin legs operated by tendons, but they are often more substantial than those found in their songbird relatives. Flightless birds, such as ostriches and cassowaries, for example, have noticeably larger calf muscles to support their greater weight.
Light songbirds are less reliant on walking. “The main way that most of them get around, at least when they need to chase something, escape from something or cover a lot of ground, is by flying,” says Stephen Brusatte, a paleontologist at the University of Edinburgh. “So the legs wouldn’t have to be so muscular.”
In other words, when looking at birds’ feet, it’s important to remember that they look that way because of the evolutionary history of birds, a history that has been largely shaped by the adoption of air by past lineages. For most birds, especially those that fly, the muscular limb is the wing, while the legs tend to appear weak in comparison.
“It’s fair to say that the extremes of living birds were not reached until after the origin of flight,” says Clarke.
Whatever their origin, those spindly legs are now a central part of modern birds’ appeal and have helped inspire nicknames and taxonomies on the Internet. The apparent thinness of the birds’ legs emphasizes the roundness of their bodies and creates the cheerful-looking leaps or rapid runs that delight us so much. His legs may look strange, but we wouldn’t have it any other way.