February 10, 2024<\/p>\n
4 min read<\/p>\n<\/div>\n
The songbirds in your backyard hop around on such itty-bitty legs. Here\u2019s why bird legs are so skinny and how they can support a bird\u2019s weight<\/p>\n<\/div>\n
By Asher Elbein<\/a><\/p>\n Splendid Fairy-wren (Malurus splendens<\/em>).<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/div>\n A bird in flight is poetry; a bird on the ground presents a conundrum. Watch a sparrow or other songbird bobbing and scratching around the forest floor and it\u2019s easy to wonder: How do they support their weight on such skinny little legs?<\/p>\n The first reason bird legs look strangely thin is that the rest of their body is mostly covered by feathers, which can dramatically magnify<\/a> its apparent size, says Julia Clarke, a paleontologist who studies bird evolution<\/a> at the University of Texas at Austin. Birds\u2019 relatively meatier thighs are tucked underneath their feathers, and most birds have scaly lower legs, Clarke says, which makes these limbs look oddly small and naked. Birds that have feathers down to their toes\u2014such as owls<\/a> and some domestic chickens\u2014appear to have thicker shins than their bare-legged cousins.<\/p>\n But it\u2019s not just a visual illusion: bird legs have some genuine differences from legs of other living animals. Birds are descended from theropod dinosaurs<\/a>, a group of bipedal animals that ranged in size. Some were as small as modern sparrows, while others were huge, such as Tyrannosaurus rex<\/em><\/a>. Whereas most mammals that can walk bipedally\u2014such as bears, chimpanzees and humans\u2014stand on their heels, dinosaurs walked on their toes and the front ball of the foot, with the remainder of their foot bones elongated and held off the ground. Living birds kept some of these features. As they took increasingly to the air, however, birds developed some divergent traits. Bird relatives such as Velociraptor<\/em><\/a> had relatively short thighs above their long shins and toes, but they held those thighs more or less vertically, like ours. Travel down the bird family tree toward modern families, however, and those thighs gradually change orientation. \u201cIn birds, that upper leg bone is held mostly horizontally, and it\u2019s kind of tightly bound into muscle groups that attach to the trunk of the body,\u201d Clarke says.<\/p>\n If you’re enjoying this article, consider supporting our award-winning journalism by subscribing<\/a>. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.<\/p>\n Why did this shift occur? Maintaining balance as a biped is a tricky business, as anyone watching a young toddler walk can attest. Without a strong center of weight supported by robust legs, any two-legged animal risks toppling. Bipedal dinosaurs kept on their feet by distributing weight across long necks and bony tails. But as bird ancestors increasingly took wing, Clarke says, the development of powerful flapping chest muscles in birds shifted their center of mass toward the front of the body. In order to maintain balance, bird torsos shrank. Their thighs shifted horizontally and became bound to the body, effectively becoming part of the hip. The knee, in turn, became the highest moving part of the leg.<\/p>\n Birds developed other anatomical tricks as well. Over time, the three metatarsal bones found in dinosaur feet transformed to a single, fused bone in birds that was both light and strong. Birds developed muscular changes as well, Clarke says, and most of their leg muscles became consolidated in the thigh, which is joined to the torso. The lower legs and feet, meanwhile, are operated by bundles of superthin tendons that run from the knees to the toes.<\/p>\n That same tendon system locks bird toes in position while they perch, allowing them to sleep without falling. \u201cNo other animal has this tendon system that allows them to locate the strong muscle action close to the body,\u201d where it\u2019s hidden under the feathers, Clarke says.<\/p>\n Not all birds have stick-thin legs, of course. Birds that spend a lot of time on the ground\u2014or make a living by grabbing prey\u2014still have characteristically slender tendon-operated legs, but these are often more substantial than those found on their songbird relatives<\/a>. Flightless birds such as ostriches and cassowaries<\/a>, for example, have noticeably larger calf muscles to bear their greater weight.<\/p>\n Lightweight songbirds are less dependent on walking. \u201cThe primary way most of them get around, at least when they need to chase something or escape from something or cover a lot of ground, is by flying,\u201d says Stephen Brusatte<\/a>, a paleontologist at the University of Edinburgh. \u201cSo the legs wouldn\u2019t need to be so muscular.\u201d<\/p>\n When looking at bird legs, in other words, it\u2019s important to remember that they look the way they do because of birds\u2019 evolutionary history, one that\u2019s been heavily shaped by past lineages\u2019 embrace of the air. For most birds\u2014especially those that fly\u2014the muscular limb is the wing, while legs tend to look puny by comparison.<\/p>\n \u201cIt\u2019s fair to say that the extremes of living birds [weren\u2019t] reached until after the origin of flight,\u201d Clarke says.<\/p>\n Whatever their origins, those skinny legs are now a central part of modern birds\u2019 appeal and have helped to inspire Internet nicknames and taxonomies<\/a>. The apparent thinness of birds\u2019 legs emphasizes their body\u2019s roundness and creates the cheerful-looking hops or darting runs that so delight us. Their legs might look odd, but we wouldn\u2019t have it any other way.<\/p>\n<\/div>\n![\"Small](\"https:\/\/static.scientificamerican.com\/sciam\/cache\/file\/F6ADC90D-A934-4F57-8D6A3BBE7766C6F5_source.png?w=600\")
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