In 1799, the famous German naturalist Alexander von Humboldt, during his trip to South America, visited a large cave in northeastern Venezuela near the city of Karipe, which the locals called the “fat mine”. In it, from time immemorial, the Indians collected “fat crops” in it, in other words, they caught incredibly fat chicks of birds, many living in this cave. These birds were subsequently described by Humboldt under the Latin name Steatornis caripensis ("fatty birds from Karipe"). The Spaniards called these birds “guajaro,” which means “crying,” or “moaning,” because of the piercing screams they make.
Guajaro - large, chicken-sized, orange-brown birds with a wingspan of about 100 cm. Guaharo's mouth resembles that of a goat’s mouth, but its large beak resembles that of a hawk.
Humboldt’s report contained two points that subsequently attracted Griffin’s attention: firstly, the birds nested and flew in the dark compartments of the cave, and secondly, the sharp, piercing sounds made by guajaro, reflected from the stone walls of the mountain, gave rise to echoes far beyond outside the cave.
In 1953, Griffin, together with the ornithologist Phelps, went to Karipa to check on site the hypothesis that he had arisen about the possibility of using “ebullient birds” of echolocation when flying in the dark.
During observations in the caves, two main tasks were set: to find out the participation of the visual analyzer in the orientation of these birds and try to establish the nature of the sounds made by them when flying in the dark. To check the first of them in the most remote cave of the cave (650 m from the entrance), where a large number of birds nested, the researchers put out the lights and for 25 minutes. remained in the dark. Even after the full adaptation of vision in the dark, not one of them found a glimmer of light. Not satisfied with this observation, they installed a camera with an open diaphragm directed towards the exit from the grotto, and exposed film frames from 5 to 9 minutes. When the film was developed, it became clear that the light did not penetrate deep into the cave. Despite this, the guajaros showed lively activity there. Annoyed by the presence of strangers, they flew in complete darkness through the cave hall, relentlessly emitting sharp, unpleasant screams. These observations showed that vision cannot play a significant role in the orientation of these birds when flying in the dark.
Secondly, it was necessary to find out to what extent the cries of birds can serve them for orientation. For these purposes, a condenser microphone, amplifiers, a portable tape recorder with a passband from 50 Hz to 15 kHz and a cathode oscilloscope with a set top box were installed in the cave. Guaharo shouts recorded on a tape recorder were of several types. Some of them were undoubtedly the callsign of birds sitting on the ledges of rocks, others - sharp, piercing - emitted excited birds flying over the researchers. But sometimes completely different sounds stood out that resembled a quick drum roll and consisted of a series of separate clicks.
At dusk, the guajaros began to fly out. The sound pattern has changed dramatically. Instead of the unpleasant piercing screams that the birds emitted inside the cave when flying out of the gathering darkness, there was an even stream of sharp, rapidly repeating clicks. Only occasionally, one of the hundreds of flying birds emitted a continuous signal, similar to those sounds that were heard in the depths of the cave during the day.
Typical oscillographic records of these clicks are shown in Fig. 102. It turned out that while the shape of the click-pulses to some extent changed from record to record, the duration, frequency of the pulses and the intervals between them remained approximately constant.
The duration of these pulses was about 1 ms. (extreme values 0.3–1.5 ms). A single click consisted of several sound waves having a frequency of about 7 kHz. No ultrasonic components in pulses were observed.
Guajaro emitted impulses in short bursts of series. The number of pulses in the series ranged from 2 to 6 or more. The intervals between pulses in the series averaged 2.6 ms. (1.7-4.4 ms.). The intensity of the click pulses was not measured, but the author notes that they were audible at a distance of 100 steps.
Thus, it was found that guaharo during the flight in the dark produce loud clicks with a low filling frequency lying within the hearing of a person. But it remained to be proved that these sounds are used by birds for echolocation purposes. For the final solution of this problem, 3 birds were caught and each of them in turn was given the opportunity to fly several times through a dark room. The birds were perfectly oriented in the dark and avoided a collision with the walls of the room. After that, their ear canals were tightly plugged. In each case, a bird with closed auditory canals flew up to the opposite wall and hit it with noise. However, orientation was restored again after removal of the ear plugs. In both cases, the birds emitted loud clicks almost continuously.
The situation changed when the light turned on in the room. The clicking sounds almost completely stopped, but even more striking was the fact that birds with plugged ears in the light did not lose their ability to detect obstacles and avoid them. Obviously, in the light of guajaro they were guided by sight.
Guajaro are fertile birds and feed mainly on stone fruits, usually of different varieties of palm trees. Obtaining fruit hardly requires the participation of echolocation, therefore it can be assumed that the sound location of guaharo is used only in time to detect an obstacle during night flying for food and successfully navigate in the dark of deep caves that serve as their place of day refuge.
E.S.AIRAPETYANTS A.I. KONSTANTINOV. Sonar in nature. Publishing House "SCIENCE", LENINGRAD, 1974