Ask SIB: “Why do Birds Knees Bend Backwards?”

Wood Stork – photo by Alan Fink


This week, while birding on Ocean Winds, my group saw a Wood Stork sitting in a strange position as if their knees were bent in the wrong direction.  Can you explain?


One of the lesson’s I enjoyed teaching to the PA Master Naturalist classes was on animal structure. In the world, certain structures exist throughout the animal kingdom. The premise being that the original design has been shaped and altered to meet the needs of animals, including humans. The term is homologous structures. 

The Dictionary of Biology describes the term as: “Homologous structures are organs or skeletal elements of animals and organisms that, by virtue of their similarity, suggest their connection to a common ancestor. These structures do not have to look exactly the same, or have the same function. The most important part, as hinted by their name, is that they are structurally similar.”

To understand this image of the Wood Stork, one must look at your own feet and legs. On your foot, you have toes that bend up and down. Then note the bones of the foot which don’t bend. Next comes the ankle to provide mostly up and down motion, but it cannot bend back as far as it can up. (Go ahead, I give you permission to wiggle your toes and feet.) From there it is more solid bone up to the knee which flexes backward. Finally, more solid bone to the hip. 

Next time you eat a chicken leg and thigh, notice how the joints move. The thigh is homologous to your thigh. The joint connecting the thigh to the chicken leg would be the knee and the meat on the leg would be the calf. Since little meat exists on the foot, it used to be you didn’t get chicken feet as part of your meal unless you are very poor. You can now find gourmet recipes, but I’ll pass. 

Look at this Wood Stork image. Note the toes are what a bird stands on. The foot holds birds up. The ankle in this picture still rests on the ground. The knee hides under the feather. While this position looks uncomfortable to us, it is the natural arrangement of bones in birds.

Wood Storks sitting near a pond on Seabrook Island – photo by Bob Mercer

– Bob Mercer, SIB’s “Resident Naturalist”

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