Our Window on Nature

Why don’t perching birds fall out of trees when they take a nap? Or off of phone lines when the wind starts to whip the bird up and down? Birds may seem to have a much better sense of balance than the rest of us, but the real answer is in the structure of their legs and feet.

Watch one of your local birds the next time you see it about to land on a branch. The first thing you’ll notice is that the bird stretches out its legs just before touching down. And what may happen too fast to observe is the bending of the leg after landing. These two actions are the secret to what at first glance seems like a remarkable balancing act.

There are flexor tendons that run down the bird’s legs, and when the leg is straight, the tendon is more or less relaxed. But when the leg is bent upon landing, it stretches these tendons, which in turn clench the bird’s toes, causing them to lock around the branch or wire.

And falling asleep doesn’t change the grip, since the weight of the bird keeps the leg in the locked position. When the bird is ready to leave it’s perch, it straightens the leg and relaxes the tendon. Come to think of it, that still seems like a remarkable balancing act.