Why Are Birds' Knees Backwards?

The animal kingdom is rich with anatomical variations, filling the earth with a wealth of biological diversity that makes each visit to the local zoo or walk in the woods a sensory wonder. Human observers process evidence through the familiarity of their humanity, comparing animal features to their own features. Such is the case of bird legs which, at first glance, appear to bend in the ̶0;opposite̶1; direction of human knees.

  1. Not the Knee

    • A bird's mid-leg joint is not a knee -- it's an ankle.

      The idea that a bird's knee moves backward, in the opposite direction of a human's forward-bending knee, is actually a misconception: Bird knees move just like human knees. While many would question this claim, noting plain-sighted evidence to the contrary in any avian observation, the fact is that no animal has a backward-bending knee, as the joint we commonly assume to be a knee is an ankle. The notion that a joint positioned at a leg's halfway point is a knee is a comparative conclusion drawn through the human experience of having a knee joint in the middle of the leg.

    The Actual Bird Knee

    • A bird's knee is high up on the leg, buried beneath feathers.

      While we can easily observe this center-positioned ankle, a bird's knee isn't so easily spotted, as it's positioned high on the leg and buried under feathers. The top bone of a bird's leg is the femur, just like a human's thigh bone, but much shorter -- picture the drumstick in a fried chicken bucket for a visual image. When a chicken walks, we easily note the backward-bending ankle, but beneath all those feathers a very familiar, forward-bending knee is also present.

    Why the High Knee?

    • A mid-leg ankle with high knee is common in many animals.

      A knee positioned high on the leg and close to the abdomen is common in the animal kingdom -- in fact, our mid-leg knee joint begins to seem like an abnormality when compared to other species' legs. Animal legs evolved to meet biological and environmental needs. In four-legged animals, rear legs with a high knee enable the creature to distribute body weight evenly as he walks and runs. In birds, who walk on two legs but lay eggs and exhibit a whole range of non-mammalian behaviors, a knee high on the leg serves other purposes.

    Advantages of a Bird Leg

    • A bird's legs allow her to sit on a nest of eggs.

      A bird's leg is perfectly adapted to two crucial avian behaviors: egg laying and flying. A leg bisected into two equal lengths by the ankle joint, coupled with a short femur bone that can flatten against the body, lets a bird lower herself straight down to the ground to lay an egg and then sit on the egg while maintaining perfect balance. This same folding pattern is employed during flight, when a bird pulls her legs to lay flat against her body, streamlining her body to minimize drag as she moves through the air.