It was once thought that homing pigeons use visual landmarks to help in their navigation, but experiments in which the pigeons’ eyesight was reduced using frosted contact lenses showed that’s not correct. Other experiments demonstrate that homing pigeons can sense the earth’s magnetic field, but many of those same experiments also show that disrupting that sense doesn’t always end up leading the pigeons astray. In addition, some experiments indicate that homing pigeons use the position of the sun in the sky to orient themselves, but that can’t be the entire explanation, either, because pigeons can navigate even on very cloudy days. It seems, then, that pigeons use a wide variety of strategies to navigate their way home.2
The mystery of homing pigeon navigation deepened back in 1997, when the Royal Pigeon Racing Association decided to celebrate its 100th year anniversary by releasing 60,000 pigeons in the south of France. These pigeons came from homes throughout southern England, and they were expected to be able to reach those homes in a few hours. While a few thousand of them ended up returning to their homes over a period of a few days, most never made it back.3
Over the years, several hypotheses have been put forth to explain why those pigeons never made it home. One of them suggests that the pigeons’ navigation was disrupted by the sonic boom of the Concorde jet whose flight path crossed that of pigeons. A recent study by Dr. Jonathan T. Hagstrum adds some support to this hypothesis.
As a geophysicist, Hagstrum was intrigued by the idea that the Concorde jet might have disrupted the pigeons’ ability to navigate. He knew that the Concorde jet was a source of infrasound – sound waves that are well below the frequency that human ears can detect. Even though humans can’t detect them, many animals can. He knew that infrasound permeates most of the earth’s atmosphere, because it is produced by all manner of natural phenomena such as earthquakes, volcanoes, avalanches, severe weather, ocean waves, and meteors. Because of the way the sound waves interact with the land, this infrasound produces patterns that are characteristic of the terrain. He wondered if the birds were using these patterns to “see” the terrain and thus navigate back home.
To find out if he was right, Hagstrum examined the release logs for pigeons that were raised at Cornell University in Ithaca, New York. They were regularly released from three locations in the state of New York, and they had no trouble finding their way home from two of them. However, one location (the Jersey Hill fire tower, 119 kilometers west of their home) proved very troublesome. Most pigeons that were released from that location never made it back. However, on one day (August 13, 1969) they all made it back from that location.
He used computer models to recreate the infrasound that would have existed around the three release sites each day the pigeons were released. His model showed that the terrain around the Jersey Hill fire tower interrupted the normal transmission of infrasound. However, on one day (August 13, 1969) there was a weather effect that cancelled the terrain’s disruption of infrasound, allowing it to propagate normally. Since that was the one day all the pigeons released from the Jersey Hill fire tower made it home, he suggests that pigeons must use infrasound as part of their navigational strategy.4
In other words, pigeons might be hearing a map that helps them figure out where they are and where they need to go! That’s incredible, isn’t it? Hagstrum suggests that other navigating animals might use infrasound maps as well, so there is probably a lot more to learn about this issue.
What I find most fascinating about this is that pigeons obviously have a combination of senses that allow them to navigate. Scientists recently learned that salmon also use a combination of senses (smell and magnetic field) to find their way back home to reproduce. I suspect that as we learn more about migratory animals, we will find that they all have multiple means by which they navigate. After all, any well-designed system is going to have backups and failsafes so as to provide optimum performance under a variety of conditions.
1. Andrew D. Blechman, Pigeons: The Fascinating Saga of the World’s Most Revered and Reviled Bird, University of Queensland Press 2006, p. 4
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2. Michael W. Eysenck, Psychology: A Student’s Handbook,Psychology Press 2000, pp. 242-243
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4. Jonathan T. Hagstrum, “Atmospheric propagation modeling indicates homing pigeons use loft-specific infrasonic ‘map’ cues,” Journal of Experimental Biology 216:687-699, 2013
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