This Bird Can Find Large Objects in Total Darkness

two brown birds with white speckles doze off with a black background while sitting on a rock

Biological Strategy

Oilbird

Mary Hoff

Image: Patty McGann / Flickr / CC BY - Creative Commons Attribution alone

Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Send Sound Signals

Many living systems send auditory signals to communicate with others, including to attract, announce, or warn. These sounds must be audible to the intended recipient in a variety of conditions, such as in wind, water, and solids. As a result, each living system has specific sounds best adapted to its environment. For example, some birds live in loud habitats, like those alongside noisy streams or in windy areas. These birds use sound frequencies that can be heard by other birds over competing environmental noises.

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Navigate Through Air

Although free from many of the physical obstructions found on land and in water, organisms that move through the air must still avoid hitting each other and objects in their flight path, such as trees and mountains. They also need to navigate from one place to another, which presents a different challenge. They use one strategy to navigate around obstructions and another to move towards their destination. For example, bats use echolocation to detect both obstacles and prey. To navigate during migration, bats also use vision, sun orientation, and likely other strategies.

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Cooperate Within the Same Species

A species is a group of organisms capable of breeding to produce fertile offspring. When individuals within a species undertake activities that benefit one another, that cooperation benefits not just the individuals, but also local and wider populations of that species. Collaborating in activities such as finding food, controlling parasites, and protecting from predators enhances overall survival. To cooperate, there must be enough benefit to an individual, and especially to a population, to justify any risk taken by individuals. Through cooperation, risk is distributed among individuals and the benefits to the community are greater than the sum of the benefits to individuals. An example is birds that gather in flocks, a cooperative activity more beneficial than acting alone. While some birds forage, others watch for predators. When one finds food, others also feed on it. Despite potential short-term competition for an individual that finds food, over the long-term, that same individual benefits when others find different food sources.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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While bats use high-pitched, inaudible sounds to find tiny insects, oilbirds use lower-pitched, audible sounds to sense walls, fruit, and other large objects.

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Introduction

Living in a pitch-dark cave is a superb strategy for a bird seeking to avoid predators that hunt by sight. But it creates a second survival dilemma: How to avoid debilitating collisions with walls, ceilings, and other birds?

The oilbird has a sound solution—literally. An inhabitant of forests in South America and Trinidad, this big bird roosts and nests in caves and emerges at night to forage on fruit in nearby trees. While finding its way around in the deep dark of a cavern—and sometimes when it’s foraging on a moonless night—the bird contracts its respiratory system in a way that allows it to emit quick bursts of audible clicks. The sound waves bounce off objects, returning to the bird’s ears in a way that allows it to determine the objects’ sizes and locations and so avoid smashing into them.

The Strategy

Unlike bats, which use short-wavelength ultrasound to detect tiny insects, oilbirds use clicks that are in the range that’s audible to humans as well as to themselves. That means the sound waves they produce are only good for detecting larger objects such as other birds and fruit, perhaps down to the size of a grape. But it clearly is good enough to meet their needs for navigating without the aid of sight.

The clicking sound an oilbird makes is the result of contraction of two sets of muscles in its syrinx—a set of membranes in the bronchial tubes of birds leading from the trachea to the lungs. The tissue around each branch of the syrinx contracts, folding a membrane inward. The bird then exhales a puff of air past the membrane and contracts the second set of muscles, creating the click. It makes the clicks in small bursts of two to seven individual sounds, taking “mini-breaths” as needed to move the air past the clicking mechanism without requiring a full inhale. The echoes return to the bird’s ears at different levels of loudness and intensity.

The larger the object, the more sound waves are deflected. This enables the bird to identify the size, shape, and location of the obstacle. Short clicks rather than loud, prolonged sounds reduce interference between the sound the bird is making and the echo bouncing back, making it possible to detect objects nearby as well as far away.

Listen to audio of sounds the oilbird makes.

The Potential

Humans already use audible-range echolocation in certain contexts. Some sight-impaired individuals use it to find their way around, clicking as they move and listening for how the sound is deflected by objects they wish to approach or avoid. It’s also used in technology, usually for underwater navigation. The oilbird’s lower-frequency sonar might provide guidance for additional applications though, such as drone navigation in the air, or even robot-assisted surgery.

The oilbird’s mechanism for making rapid sounds could provide valuable insights for controlling sound or other vibration production within narrow specifications. This might be applied to anything from creating new musical instruments to transforming the noise from wind turbines.

Off-Target Clicks Help Fruit Bats Find Food

Egyptian fruit bat

These fruit-eating bats home in on a meal by aiming sound waves to the side of their intended target, then adjusting their path to reduce the difference in timing between the waves bouncing back.

a bat with large ears and a large nose hangs upside down on a dray structure at night

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Nose Structure Aids Echolocation

Ghost bat

The nose leaf of false vampire bats may be used in echolocation, though its role is not well understood.

Biological Strategy

Clicking Noises Interfere With Predators’ Sonar

Grote's bertholdia moth

Clicking noises released by the tiger moth interfere with predatory bat sonar signals by distorting the echo signature.

Last Updated March 24, 2020

References

“We propose that echolocating birds use click-type signals for echolocation because they are short in duration, permit-ting detection of objects even at very short distances (i.e., with no overlap between signal and echo). At the same time click-type signals do not require the laryngeal specializations observed in bats necessary to produce a sufficiently short frequency-modulated signal. … Additionally or alternatively, clicks may be more effective biosonar signals for detection of objects at greater distances because they may be (i) less energetically expensive to pro- duce using the syrinx and (ii) louder than other signal designs using the same energy input.” (Brinkløv et al. 2013:123)

Journal article

Echolocation in oilbirds and swiftlets

Frontiers in Physiology | 5/28/2013 | Signe Brinkløv, M. Brock Fenton, and John M. Ratcliffe

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Reference

“Both sonar clicks and agonistic squawks are initiated by contraction of the sternotrachealis muscles (Figs. 7 and 16) which stretch the trachea, reducing the tension across the syrinx and causing the cartilaginous bronchial semi-rings supporting the cranial and caudal edges of the external tympaniform membranes (ETM) to hinge inward, folding the ETM into the syringeal lumen (Fig. 17). Bernoulli forces created by expiratory air flowing through the restricted syringeal aperture presumably initiate vibration of the internal and/or external tympaniform membranes.” (Suthers and Hector 1985: 243)

Journal article

The physiology of vocalization by the echolocating oilbird, Steatornis caripensis

Journal of Comparative Physiology | March 1985 | Roderick A. Suthers and Dwight H. Hector

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