How do birds manage to sleep on one leg?

We see cranes in shallow water sleeping for hours on one leg. And birds roost in dense trees where they hike up one leg, turn their head into their feathers and fall asleep on a twig. How do they avoid falling over? Have you tried balancing on one leg for a long time with your eyes open? Then try it with your eyes shut? The birds are even better than that because they are asleep at the same time. No conscious effort is applied in keeping balanced. It is all automatic.

Cranes sleep on one leg
Cranes sleep on one leg. Image: MikeB based on one by Jay Goodrich.

I’ve concluded that birds can achieve this feat of balance because they “have two specialized balance-sensing organs: the vestibular system of the inner ear and an additional balance sensor located between the hips called the lumbosacral organ” (study: “A Physical Model Suggests That Hip-Localized Balance Sense in Birds Improves State Estimation in Perching: Implications for Bipedal Robots”).

Here is some more about this special avian organ which is a reworking of text from a study by an AI computer (ChatGPT).

The extraordinary ability of birds such as the Sandhill Crane in New Mexico (as shown in the picture) to sleep on one leg is probably due to the lumbosacral organ (LSO), which is a set of modifications in the lower spine of birds (Aves). These modifications are found in a structure called the synsacrum, which is a fused region of lumbosacral vertebrae. The LSO includes transverse canal-like recesses in the synsacrum that align with the lateral lobes of the spinal cord, as well as a dorsal groove in the spinal cord that contains an egg-shaped glycogen body. Some researchers believe that the LSO may be a secondary balance organ, similar to the semicircular canals in the inner ear, which are important for maintaining balance in mammals. However, this hypothesis is based largely on observational data, and the LSO has received relatively little attention in recent research.

I’ve said ‘probably due to’. The point there is that I can’t find any hard evidence that the LSO is the wonder organ which allows birds to sleep on one leg. There are two added factors. The website ‘BirdNote’ states: “And by standing on one leg, a bird reduces by half the amount of heat lost through unfeathered limbs”. And the Audubon Society states: “A bird’s weight pressing down on its heel tightens a tendon in the heel, which in turn forces the toes to close in a very tight grip on the twig.”

There is quite a lot of sophisticated anatomy at play which allows birds to sleep for hours on one leg without falling over which is what would happen if humans tried it. For the sake of clarity, humans have one balancing organ, the vestibular system in the inner ear which includes the semi-circular canals filled with cilia (fine hairs) and fluid. They send signals to the brain on the human’s orientation and movement.

Below are some more articles on birds.

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Post Category: Birds > anatomy