Birds have internal compasses that can determine directions from the sun’s position during the day, star patterns at night, and Earth’s magnetic field. A compass can guide birds to a region, but not to particular sites within it. For that, birds need a map. But science has never agreed on an acceptable navigational map.
To function as a map, an environmental cue must be present in a gradient form, so birds can differentiate between levels or amounts in different areas. For example, the odor of an atmospheric gas might be greater at one place than another, or the intensity of the magnetic field might be stronger here, weaker there. Birds must have a map stashed somewhere in their brain that shows gradients of an environmental cue (or cues) as a spatial network that can be interpreted and followed.
In recent years, two environmental cues have been proposed as bases of a navigational map. The first utilizes olfaction to detect odors from volatile hydrocarbon gases in the atmosphere.
In the early 1970s, Floriano Papi and his colleagues, working in Italy, released 20 homing pigeons away from their loft. Ten had their olfactory nerves rendered inoperative, and 10 had functional nerves. All of the birds with functional nerves returned to the loft, while none of the birds with inoperative nerves returned. Other research supported the use of olfaction and odors.
Papi proposed that pigeons learn the odors of volatile atmospheric gases under different wind directions at their home lofts, including the odors’ relative strengths. When displaced, pigeons compare the local odors and wind direction with patterns they know from home.
Papi’s olfactory map was a hard sell. While researchers agreed that his experiments were repeatable, they couldn’t wrap their heads around the concept of birds navigating great distances based on atmospheric gases. During the early 2000s, that changed, and some disbelievers decided that the olfactory map was real.
A second environmental cue, infrasound, was suggested in the 1960s but lacked support. Then, in 2013, it was reactivated and proposed as the basis of a navigational map by Jonathan T. Hagstrum, a geophysicist with the U.S. Geological Survey in California.
Hagstrum is curious to a fault. In the 1970s, as an undergraduate at Cornell University, he heard William T. Keeton give a lecture about his research on homing pigeons. Keeton, a brilliant investigator who died at age 47 in 1980, described how his birds homed with great accuracy from most release sites but consistently had problems with three near Ithaca, New York — Castor Hill, Jersey Hill, and the town of Weedsport. Keeton’s hypothesis was that pigeons utilized Earth’s magnetic field, and he didn’t think magnetic anomalies at the sites were the problem. He asked the geologists for ideas.
Several years later, Hagstrum read that pigeons can hear infrasound — sounds less than 20 Hz, below what humans can hear — and thought he had an answer for Keeton. Infrasound, he said, is generated by minute vibrations caused by deep ocean waves, and can be propagated thousands of kilometers through Earth’s surface and the atmosphere. Perhaps the pigeons were listening for the low-frequency rumble from their loft area — waves from the vibrating loft itself or physical features nearby. He knew that under certain topographic and atmospheric conditions, infrasound can be shifted and might not be available to the birds.
Read it yourself
Geophysicist Jonathan Hagstrum’s research paper about homing pigeons and infrasound is available online.
Jonathan T. Hagstrum (2013) Atmospheric Propagation Modeling Indicates Homing Pigeons Use Loft-specific Infrasonic ‘Map’ Cues. Journal of Experimental Biology 216: 687-699; doi: 10.1242/jeb.072934.
Hagstrum and a colleague developed a complex computer program and reconstructed the terrain and atmospheric conditions for all of the days when Keeton’s pigeons had failed to home from the three Ithaca-area sites. On each day, Hagstrum’s model indicated that infrasound bent up, creating an acoustic shadow, so the infrasound cue was not available. It explained every failure. More important, it explained why, one day, all the pigeons arrived at a site where they had almost always failed to return previously. For that site, the model produced a nearly constant acoustic shadow. But on August 13, 1969, the program showed that a temperature inversion had caused the infrasound waves to be present and detectable.
Hagstrum offered anecdotal data that also support the infrasound argument. Most homing-pigeon races result in a very high percentage of finishers, say 95 percent. A low number of finishers would suggest a navigation problem, perhaps interference in the homing mechanism.
He studied four homing-pigeon races in which the number of finishers was disastrously low. In all four, the birds’ flight paths fortuitously intersected the flight paths of Concorde supersonic transports. The jets laid down sonic-boom carpets that extended to the ground, blotting out any infrasound the birds were trying to pick up.
While the avian navigational map is not yet agreed upon, a number of researchers are looking seriously at olfaction and the odors of atmospheric gases. Jonathan Hagstrum has made a strong case for hearing and infrasound. We’ve yet to learn how researchers will respond to his proposal, as it’s been out for only three years. However the questions are resolved, the ability of birds to find their way home remains truly amazing.
This article from Eldon Greij’s column “Amazing Birds” appeared in the January/February 2017 issue of BirdWatching.
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