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

This undated electron microscope image made available by the U.S. National Institutes of Health in February 2020 shows the Novel Coronavirus SARS-CoV-2, yellow, emerging from the surface of cells, blue/pink, cultured in the lab. Also known as 2019-nCoV, the virus causes COVID-19. THE CANADIAN PRESS/AP, NIAID-RML
COVID at a Glance for Monday, Dec. 6

Numbers come from reports from the City and Borough of Juneau Emergency… Continue reading

It's a police car until you look closely and see the details don't quite match. (Juneau Empire File / Michael Penn)
Police calls for Tuesday, Dec. 7, 2021

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

Pearl Harbor survivor and World War II Navy veteran David Russell, 101, reads a birthday card while talking about his time aboard the USS Oklahoma and his life after the war on Monday, Nov. 22, 2021, in Albany, Ore. (AP Photo/Nathan Howard)
101-year-old returns to Pearl Harbor to remember those lost

By Audrey McAvoy and Gillian Flaccus Associated Press HONOLULU — When Japanese… Continue reading

This photo taken on Dec. 1 shows the sun at Mendenhall Campground. (Courtesy Photo / Deborah Rudis)
Wild Shots: Photos of Mother Nature in Alaska

Reader-submitted photos of Southeast Alaska.

This undated electron microscope image made available by the U.S. National Institutes of Health in February 2020 shows the Novel Coronavirus SARS-CoV-2, yellow, emerging from the surface of cells, blue/pink, cultured in the lab. Also known as 2019-nCoV, the virus causes COVID-19. THE CANADIAN PRESS/AP, NIAID-RML
COVID at a Glance for Friday, Dec.3

Numbers come from reports from the City and Borough of Juneau Emergency… Continue reading

A joint investigation between the FBI and Canadian law enforcement agencies resulted in the arrest of a Canadian man for cybercrimes on Nov. 30, 2021. (Michael Penn / Juneau Empire File)
Canadian man indicted in international cybercrime case

His attacks targeted State of Alaska computers as well as Canadian ones.

This photo available under the Creative Commons license shows a gynandromorph of a common blue butterfly. Gynandromorphy, meaning female-male-morphology, is well-known, apparently, among birds, including chickens and several songbirds of the eastern U.S.; these individuals have one half with male plumage and the other half with female plumage. They also occur in reptiles, amphibians and fishes (as well as a variety of insects and other invertebrates.) (Courtesy Photo / Burkhard Hinnersmann)
On the Trails: Determination of biological sex —it’s a complex topic

The determination of biological sex is a complicated matter, even just focusing on vertabrates.

Most Read