Can Pigeons "Hear" Their Way Home?

Abstract

Julius Caesar used Homing Pigeons to carry messages from the remote corners of Europe to home, in Rome. That was two thousand years ago. Ever since that time, thoughtful people have wondered at these amazing birds, and in the present century, this wonder has manifested itself in some honest scientific study, trying to learn just how they do it. What "super-human powers" do the pigeons possess that enable them to take off in unfamiliar territory, make a few circles, and fly home with remarkable speed?

Scientists in Germany, England, Italy, Switzerland and the United States were probing at this question. Then in 1967, Dr. William T. Keeton in the Avian Orientation Research Project, at Cornell University, Ithaca, New York, turned his attention to the question. For many years Cornell had the reputation of having one of the world's leading centers of research in animal behavior - everything from honeybees to monkeys. Keeton was a "natural" - not only because of his boyhood interest as a pigeon fancier, but - because of his broad, multidisciplinary background in many areas of science, besides biology. In addition to his formidable reputation as one of the most popular professors at Cornell and his recognition in the scientific world, Keeton had the ability to get the funds he needed to do the work at hand. This was vital to sustain a program on pigeons that would take many years to show some results.

As most readers already know, birds are basically visual (eye-brained) animals. They use their eyes to find food, to detect danger, and to receive certain mating stimuli. So the research under Keeton's direction started by exploring how the pigeons use their eyes to find the signals for orientation and navigation. The first ten years of his project were involved mostly with visual stimuli, but a few other "powers" entered the picture along the way.

Using pigeons from the established

strains of Racing Homers in America, Keeton's team learned that these birds primarily use the sun to navigate. They have some kind of uncanny internal timeclock that tells them where the sun should be in the sky at any moment in the day at home. When they are released at a place where the sun is not where it should be, they seem to know how to relate this to where the sun would be at home, compare the two, and head toward home.

So much for ideal conditions, on a clear day with sunlight. Gradually, the Cornell people stripped away one visual cue after another. What happens on overcast days? What happens at night? (The researchers did train enough pigeons to home at night to complete that experiment.) What happens when the pigeons are wearing foggy contact lenses, that obscure any image beyond a few yards? They still come home. How? The pigeons can discern the polarity of light in the sky and probably still tell where the sun is, despite the overcast. Perhaps they can tie this with an ability to perceive light in the ultraviolet range and spot the position of the sun - through the clouds.

Later research showed that the pigeons are extremely sensitive to barometric pressure, extraterrestrial gravity (pull of the moon), and the magnetic north pole on earth. Still, it seemed that the explanation of the pigeon's homing ability was incomplete. After each discovery, a thorough attempt was made to strip-away the birds' ability to utilize the previously explored senses, and a significant number could still find home, almost as though they were without any handicap. They must have some other senses. A laboratory in Italy had run some convincing experiments that lead to the conclusion that pigeons use their sense of smell to find home. But, try as they may, the Cornell researchers could not replicate these experiments. This may have been due to the difference in climate or the difference in the birds used, but Cornell finally dropped

its olfactory explorations on homing pigeons. This left the sense of hearing, and they rolled up their sleeves. Are you ready?

To understand hearing, it is helpful to know a little about sound. The easiest way to think of sound is like the waves that are formed when you drop a rock in a pond. A big rock makes a high wave with a long distance between the top of one wave and the top of the next. A small rock makes a small wave with less distance between the top of one wave and the next. In sound we call the height of the wave amplitude (or loudness). The length between one peak and the next is called wavelength. This is used also to describe the rate of speed that the waves hit the shore, and it is called frequency (or pitch). In sound pitch is measured in cycles per second (waves per second). In recent years, cycles per second has been renamed Hertz (Hz) in honor of the German physicist (not the car rental company). So much for the technical jargon.

Most people can hear low-pitched sounds from about 20 to 30 Hz up to highpitched sounds of about 12,QOO Hz, depending on age and other factors. However, at the extreme ends of this spectrum, the sounds must be rather loud to be audible. (For example, the fundamental pitches from a piano range from about 28 Hz to 4186 Hz.) But what about pigeons' hearing? Earlier tests had shown that pigeons were about as sensitive as humans between 200 Hz and 10,000 Hz. Nobody had investigated the pigeons' ability to hear low-pitched sounds - really lowpitched sounds, below the range of human hearing, which are know as "INFRASOUNDS". Dr. Melvin Kreithen and Marilyn Yodlowski (an undergraduate student) tried the pigeons more than an octave below the range of human hearing - at 10 Hz. The pigeons could hear it - not with high sensitivity - but with much greater ability than humans. They went down another octave to 5 Hz. Again the pigeons responded. This was a most impressive discovery - especially, as a part of an undergraduate project.

After this, Douglas Quine (a graduate student of Keeton) began a detailed study of the homing pigeons' hearing ability at extremely low frequencies. He went down to 2.5 Hz - even to 1 Hz - and he found that the pigeons could not only hear these sounds, but they were more than 200 times more sensitive in this range than humans. Quine and Kreithen went to 0.5 Hz (one cycle every two seconds). Quine eventually went down to 0.05 Hz (one cycle every twenty seconds), and the pigeons could still hear it! It is difficult for humans to believe.