An American robin perches on a branch, with toes loosely curled. (Courtesy Photo / Bob Armstrong)

An American robin perches on a branch, with toes loosely curled. (Courtesy Photo / Bob Armstrong)

On the Trails: Why don’t birds fall off their perches?

A growing body of evidence suggests that birds have a second organ of equilibrium.

By Mary F. Willson

For the Juneau Empire

For a long time, the accepted explanation was simply that perching birds just cling with tightly curled toes. A tendon runs from the toes up the elongated foot and behind the ankle.

This diagram shows the tendon (in color) that attaches the toes to muscles higher up on the leg, passing around the ankle; when the bird bends the ankle, the tendon tightens the toes. (Courtesy Image / K.M. Hocker)

This diagram shows the tendon (in color) that attaches the toes to muscles higher up on the leg, passing around the ankle; when the bird bends the ankle, the tendon tightens the toes. (Courtesy Image / K.M. Hocker)

When a bird bends the ankle a little, the tendon tightens and pulls the toes into a curl. There’s even a locking mechanism, rather like a ratchet, that keeps the toes curled without muscular effort.

It’s quite true that perching birds can grip tightly, to hold prey or perhaps hang upside down like a chickadee, although that may take some muscular effort in addition to the useful ankle tendon. But a bird perched at rest on a twig is not usually holding on tightly; the toes are loosely curled, more or less draped over the twig, and the bird is just balanced there. It can perch there, stably, while twisting and turning its head to track what is going on around it or while holding the head steady as the body bobs up and down as the twig moves. How do they do that?

A song sparrow perched on a branch, with toes not curled (Courtesy Photo / Bob Armstrong)

A song sparrow perched on a branch, with toes not curled (Courtesy Photo / Bob Armstrong)

Birds, like most other vertebrates, have complex organs in their heads (one by each ear) that are responsible for maintaining equilibrium. The principal working parts are three fluid-filled semicircular canals (one horizontal and two vertical at right angles to each other), and two small chambers housing viscous fluid and tiny stones. When equilibrium is disturbed, the fluids move, triggering lots of tiny hair-like structures to send signals of disturbance to the brain. Then, if all is goes well, the brain manages to direct the body to restore its balance.

Sometimes, as we know from experience, that system is overtaxed. If we change position or direction of movement too rapidly, we can get dizzy and may start staggering or even fall down. Human gymnasts must somehow train their balance organs and their connections, so that doesn’t happen to them. Similarly, tree squirrels, which rapidly change direction of travel, must have ways of maintaining balance.

Birds can have this problem too. For them, it is a common problem. They may have to perch on a wobbly twig but keep the head steady while surveying their environment. Or, conversely, they may have to move the head around, keeping track of things, while the body is stationary. So why don’t they dizzily fall off their perches?

A growing body of evidence suggests that birds have a second organ of equilibrium, located near the hip. The lowest vertebrae (lumbar, sacral and a few caudal ones) of a bird’s spine are fused together into a unit called the synsacrum and to the pelvis. The probable organ of equilibrium (called the lumbo-sacral organ) is an egg-shaped glycogen (a polysaccharide) body in a dorsal groove of the spinal cord, together with a set of transverse canals in the synsacrum that align with lateral lobes on the spinal cord, and that arrangement is somehow linked to the nervous system. It’s not at all clear how this might work, but it’s best developed in birds that perch.

Having two organs of equilibrium seems to allow birds to manage head and body separately in challenging circumstances. However, more detailed research is needed to show just how this works and how it is all coordinated.

• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.

More in News

(Juneau Empire File)
Aurora forecast for the week of Nov. 27

These forecasts are courtesy of the University of Alaska Fairbanks’ Geophysical Institute… Continue reading

Jonson Kuhn / Juneau Empire File
Even the Grinch got into the holiday spirit at last year’s Gallery Walk on Friday, Dec. 2, 2022.
An abundance of traditional and new ways to capitalize on this year’s Gallery Walk

More than 50 events scheduled Friday afternoon and evening from downtown to Douglas.

This view is from Wrangell on Sept. 11, 2022. (Photo by Joaqlin Estus/ICT)
Conservation group supports formation of new Alaska Native corporations

The conservation group the Wilderness Society has changed its position and now… Continue reading

From her hospital bed on Friday, Nov. 24, Christina Florschutz demonstrates how she pulled pajama bottoms that she found in the landslide debris over her legs, arms and head to keep warm. Her house was destroyed in the landslide, and after spending the night in the wreckage, she was rescued the morning of Tuesday, Nov. 21. (Caroleine James / Wrangell Sentinel)
Elementary school aide who survived Wrangell landslide calls circumstances a miracle

Christina Florschutz trapped overnight by landslide that killed at least 4 people, with 2 missing.

Lylah Habeger (left) and Jaila Ramirez lead the Konfeta Corps during a rehearsal of Tchaikovsky’s “The Nutcracker” at Juneau Dance Theatre. The ballet will be performed in the Juneau-Douglas High School: Yadaa.At.Kalé auditorium Friday through Sunday. (Photo courtesy of Juneau Dance Theatre)
‘Nutcracker’ tradition, with a twirl of new choreography

This year’s performances feature a cast of 93, ages 5 to 78

(Michael Penn / Juneau Empire File)
Police calls for Sunday, Nov. 26, 2023

This report contains public information from law enforcement and public safety agencies.

(Michael Penn / Juneau Empire File)
Police calls for Saturday, Nov. 25, 2023

This report contains public information from law enforcement and public safety agencies.

Rain at the National Weather Service Juneau station on Nov. 11 doesn’t exist as snow until hits the upper portion of nearby Thunder Mountain. So far this November has been both warmer and wetter than normal. (Photo by National Weather Service Juneau)
El Niño playing outsize role in Juneau’s warmer temperatures, according to National Weather Service

Early peek at numbers shows Juneau is 4.9 degrees warmer than average this November.

An emergency rescue vehicle parks in front of the Riverview Senior Living center at midday Monday after resident Nathan Bishop, 58, was discovered in the attic about 40 hours after he was reported missing. (Mark Sabbatini / Juneau Empire)
Nathan Bishop found alive in attic of Riverview Senior Living complex after 40-hour search

Family members say they remain supportive of facility’s locally available assisted living services.

Most Read