4 min read

Microbaroms and Giant Fipples

Pathways of Sound
Microbaroms and Giant Fipples
Migrating birds. Photo by Michaela from Pixabay 

At this exact moment, a massive shift of energy and biomass is happening on the Earth's surface as billions upon billions of birds undertake epic migrations in response to the changing seasons.

Earth from space
It's a long journey between hemispheres. Image by Said Shurafa from Pixabay

The question of how birds, and especially young birds with no knowledge of the world, migrate vast distances with unerring accuracy has long mystified people.

If birds fly thousands of miles over unfamiliar terrain, what clues could they be using?!

migrating snow geese
How do geese know where to go every winter! Photo by Jim Drinkwine from Pixabay

I love finding and sharing fascinating nature stories with you each week, and I welcome your comments and contributions. Check out some subscription options here. Thank you for flying on this journey with me!

Personally, I am fascinated by the idea that birds might use infrasonic sound as a navigation tool. I love this idea because it points to a fundamental principle of earth-sky-ocean interactions that you would otherwise never think about.

Did you know that air currents moving relentlessly over the earth's surface create sounds as they interact with key features? These interactions push fluids (both air and water) out of equilibrium, which creates harmonic disturbances (sounds) as the fluids ripple back into equilibrium. But we don't notice because these infrasonic sounds are extremely low frequency (0.1 to 20 Hz) and outside the range of human hearing.

For example, a mountain range forces air currents to rise abruptly then sink in the lee of the range in a series of bouncing waves, called gravity waves. Another way to look at this is to imagine that wind passing over the lip of a mountain range is like blowing on the mouthpiece of a giant fipple (a wind instrument).

satellite view of clouds
Satellite view of gravity waves forming as wind flows over isolated mountain peaks.
Air flowing over mountains creates ripples of bouncing air, with a line of clouds in each bounce.

In addition, as wind passes over the ocean it creates a near-continuous hum as ocean waves interact with the atmosphere to produce low-frequency acoustic signals in the atmosphere called microbaroms.

ocean waves
Standing waves on the open ocean and waves breaking in the surf both create infrasonic sounds. Photo by Sabrina Eickhoff from Pixabay 

Why might this matter to migrating birds?

This matters because infrasonic sounds travel hundreds of miles, which means these sounds resonate outward to create a vast web of auditory landmarks.

North America image
North America as a soundspace of loud auditory cues. Photo from Wiki Images on Pixabay

Imagine you are a bird migrating from Alaska to South America: This entire route follows a coastline as well as a long, continuous series of mountain chains that includes the Cascade Mountains, the Sierra Nevada, the Sierra Madre and the Andes. Theoretically, a bird listening to the sound of microbaroms in one ear and the sound of giant fipples in the other ear would never get lost.

Map of Americas
An imaginary journey following the sounds of ocean waves and long mountain ranges. Image by David Lukas

Of course, birds use and follow a bunch of navigational clues, including magnetic compasses in their brains, polarized light projected into the sky at sunrise and sunset, and glimpses of passing landscapes. All these clues add together to give migrating birds a lot of information to work with.

migrating birds at sunset
When the sun lies over the curve of the earth it shines bands of polarized light into the sky, providing another clue that migrating birds follow. Photo by Holland Evans from Pixabay

What's most fascinating to me about migrating birds following infrasonic sounds is that this idea lies in the realm of conjecture. It's one of those amazing ideas that makes perfect sense, and explains the world in a way that I find magical, but it's never been proven.

Out of the world's roughly ten thousand species of birds, only five species have been studied for their perception of infrasonic sounds, and four of those species tested positive for hearing these sounds.

Domestic pigeons are one of the few birds known to perceive infrasonic sound. Photo by Marc Pascual from Pixabay

If the world is speaking loudly and clearly, and if birds have been migrating for hundreds of thousands of years, what do you think of the idea that birds have learned to follow these sounds?

Please help me continue this journey of sharing amazing nature stories with you.