It has been long known that homing pigeons and probably many other migratory birds can hear “infrasound” – sounds at frequencies below human ability to discriminate as pitch. Our pitch discrimination ends at around 20Hz, below which we perceive continuous sounds as a fluttering. Down to some point – maybe 3 to 4 Hz we might be able to hear pressure fluctuations (if the sound is loud enough). Further below that our hearing fails, although we may get perceptual cues in our body, like a “gut” response.
We don’t need to hear infrasound except to warn us of some large things moving our way – like stampeding herds of bison, earthquakes, or avalanches. Migrating birds on the other hand can use infrasound to let them know about the weather – signaled by fluctuations in barometric pressure in the atmosphere. It also turns out that long wavelength infrasound helps them orient to their destinations.
One source of infrasound is the pulsing of the waves along the world’s beaches. Long, heavy masses of water regularly pulse the shore causing the land to flex in and out like a giant loudspeaker cone. These extremely long wavelength sounds can travel hundreds to thousands of kilometers and are oriented to the shorelines, so a bird flying across land hearing theses sounds can orient by the angle of the wave fronts and perhaps by how they intersect with wave fronts from other shores.
An early revelation about how birds use sound to navigate came during an annual homing pigeon race across the English Channel. In the subject year the birds all came in hours later than expected. On investigation it was found that during the race the supersonic Concord airplane flew over the race, thumping its characteristic infrasonic-boom. This extremely powerful noise may have temporarily deafened the birds causing them to lose their bearings.
While there is yet clear evidence, it would stand to reason that if birds in the air have adapted to ocean wave-generated navigation cues, that migratory animals in the ocean like whales, sharks, and tuna may also be able to navigate from similar sound sources. The precautionary note here is that of the many of the noises we humans are introducing to the sea, many include infrasonic energy: from ship propellers to seismic airgun surveys, to Low Frequency Active Sonar (LFAS). While none of these sounds seem to be causing these animals to jump out of the water, we have no idea how these sounds interfere with their ability to hear where they are or find where they are going.
(This newsletter was prompted by a recent paper in J.Experimental Biology and a good summary of the findings and significance in The Atlantic)