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 photo)
Aurora forecast through the week of Nov. 3

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

Rep. Bryce Edgmon, I-Dillingham, speaks in favor of Senate Bill 48, the carbon credits bill, on Tuesday, May 16, 2023, in the Alaska House. At background is Department of Resources Commissioner John Boyle and staff supporting the bill. (James Brooks/Alaska Beacon)
Alaska House control flips from predominantly Republican coalition to mostly Democratic coalition

Preliminary election results show the new House majority will have at least 22 members.

West Juneau homes on Douglas Island late Thursday afternoon. (Mark Sabbatini / Juneau Empire)
What will Trump as president again and a more liberal Alaska Legislature mean for Juneau?

Election appears to shake up federal and state governments in different ways, leaving lots of unknowns.

Aurelie Alexander photographs a helicopter hoisting cellular equipment onto the roof of the Marine View building at midday Wednesday. As a resident of the apartment/office building, she and others were notified to leave the building during the helicopter operation. (Laurie Craig / Juneau Empire)
Hovering around downtown for better phone service

New AT&T equipment installed atop Marine View Building joins other telecom upgrades downtown.

The Columbia is scheduled to replace the Kennicott on the mainline ferry route between Southeast Alaska and Bellingham, Washington, starting in mid-December. (Alaska Marine Highway System photo)
Proposed summer ferry schedule for 2025 remains much the same, with Columbia replacing Kennicott

Public comments being accepted until Nov. 12, with virtual meetings scheduled that day.

A simulated photo shows the tailings stack and other features of Hecla Greens Creek Mine under the final notice of decision for expanding the mine announced Thursday by the U.S Forest Service. The expansion will extend the life of the mine up to 18 years. (U.S. Forest Service)
Extending Greens Creek Mine production for 12 to 18 years gets final OK from Forest Service

Agency says there will also be more habitat protection measures and mine waste disposal capacity.

A sperm whale is seen in an undated photo published by the National Oceanic and Atmospheric Administration. (NOAA photo)
Southeast Alaska fisher could get six months in prison after attempting to kill sperm whale

Federal prosecutors are recommending that an Alaska fisher serve six months in… Continue reading

Voters at Anchorage City Hall wait in line to cast their ballots on Nov. 4, 2024, the day before Election Day. City Hall, in downtown Anchorage, was one of the designated early voting sites in the state’s largest city. (Yereth Rosen/Alaska Beacon)
No leaders change as 1,500 more ballots are added to Alaska’s election count

Almost 46,000 votes cast before Election Day remain uncounted, according to absentee and early vote figures.

Most Read