Bird Families

White-bellied Swiftlet / Collocalia esculenta

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Guajaro is not the only representative of birds living in the depths of caves and using an acoustic method of orientation when flying in the dark. Swifts (genus Collocalia) are widespread in Southeast Asia, several species of which nest in dark corners of deep caves, often in the vicinity of bats. These birds, like the guajaro, have long been persecuted by the indigenous population, since their nests, built from a mixture of saliva, mud, twigs and grass, are used for food. Collecting edible nests on some islands is the main occupation of the population. The annual extermination of offspring forced swifts-swifts to arrange their nesting colonies further and further from the entrance to the depths of the caves, on the sheer cliffs and cornices of high underground grottoes.

Unlike guajaro, swiftlets are diurnal insectivorous birds. While hunting for insects, they are mainly guided by vision, but when they go to their nests in the twilight of caves, they begin to emit low-frequency, well-audible clicks. These clicks in their parameters were almost the same, but the frequency of their repetition varied, increasing as the illumination in the cave decreased and reaching a maximum when landing on the nest. Novick (1959) gives the following parameters of clicks in the Ceylon swiftlet (Colilocalia brevirostris unicolor). The duration of each pulse-click was 2-6 msec. The fundamental frequency was 4-5 kHz and was accompanied by many overtones. The full spectrum of the signal could not be obtained. The pulse repetition rate was determined from several 2-3-second records of a series of pulses, perceived by ear as a "crackle". It varied from 5.5 to 10 pulses / sec. The association of these clicks and "crackles" with echolocation was established experimentally when several specimens of these birds were examined in a laboratory room. With their eyes closed or in complete darkness, they freely flew around the room, avoiding various large obstacles, but continuously emitting clicks that merge into "crackling" when approaching a wall or other objects. With the simultaneous switching off of vision and hearing, the birds became helpless and faced any obstacle. In good lighting conditions, turning off the hearing did not affect the orientation of the swiftlets, which was carried out exclusively at the expense of vision.

Medway (1959) investigated clicks in another species of swiftlets - Collocalia maxima bowi.) During the "crackling" they emitted 5-6 impulses per second. The duration of each of them did not exceed 2.5 ms, and the filling frequency varied from 1.5 to 5.5 kHz, with two amplitude maxima at frequencies of 2 and 4.5 kHz. Oscillograms of single clicks of different individuals show the distribution of frequency peaks within the pulses. These individual pulse variations may make it easier for each bird to distinguish its own echo from the calls and echoes of many other birds flying simultaneously through the cave.

A special check showed the absence of ultrasonic frequencies in the calls of the swiftlets Collocalia maxima, C, salangana, C. esculenta (Cranbrook, Medway, 1965).

Echolocation clicks are emitted by birds only during flight, when flapping their wings. Sitting birds with folded wings do not produce clicks and crackles. The first echolocation calls in young swiftlets appear only when their wings reach the size of adult birds (Harrison, 1966).

In 1967, Medway recorded and analyzed echolocation signals from the Javan swiftlet (Collocalia fuciphaga) and found that their filling frequencies were approximately in the same range as in the previous species, between 1.5 and 4.5 kHz (Medway, 1967).In addition, it was shown that birds of this species could not avoid collisions with wooden, upright poles, 1 cm2 in diameter in the dark, but freely maneuvered between them in good lighting. The distance between the poles was 15 cm, that is, almost half the wingspan of the swiftlets (27 cm).

Better results in experiments with overcoming obstacles were obtained by Griffin and Sateris on another species of swiftlets - Collocalia vanikorensis granti (Griffin and Suthers, 1970). In a small dark chamber, the birds were sequentially presented with 3 barriers: plastic tubes (8 mm in diameter), metal rods (6.3 mm in diameter), and insulated wires (2 mm in diameter). When overcoming the first two types of obstacles, the birds showed, respectively, 68 and 80% of non-contact flights, but wires with a diameter of 2 mm caused difficulties in their detection, and the number of successful flights was only 43%. The authors explain the differences with the results of Medvey's experiments by the fact that C. vanikorensis used higher frequencies in echolocation signals, which could increase the resolution of their location systems. Most of the energy in their clicks was concentrated between 4.5 and 7.5 kHz. The spectrum of several clicks spread even up to 16 kHz. The duration of the high-amplitude parts of the signals was usually 4-8 msec, and the intervals between them averaged 116 msec, with a minimum of 48 msec. and a maximum of 358 msec., respectively, the repetition rate - 20-3 clicks per second. The researchers failed to find any regularities in the change in the parameters of location clicks in this species of swiftlet when they approach an obstacle.

So, from the available data on acoustic location in birds, it is still possible to draw only the following conclusions. First, in optically unfavorable conditions, some species of the two orders - Caprimulgiformes and Apodiformes - can use echolocation for the purpose of unimpeded flight. Secondly, the main energy in the location signals (clicks) emitted by birds is concentrated in the frequency range audible to humans and varies in different representatives from 2 to 7.5 kHz. Third, the low-frequency filling of signals determines the low resolving power of the echolocation systems of these birds, and in functional terms, echolocation in them, apparently, plays only a subordinate role with well-developed visual reception.

The issues related to the mechanisms of echolocation signals emission and the methods of perception and processing of reflected pulses remain completely unexplored. The capabilities and range of echolocation of birds in terms of such indicators as sensitivity, resolution, object recognition, etc. have not been subjected to a thorough experimental study.

Guajaro and 4 species of swift swifts are the first objects in the discovery of echolocation in birds. Can we expect this list to expand in the coming years? On this occasion, a number of assumptions have been made. It is almost certain that echolocation will be found in several other species of cave swift swifts.

Noteworthy is the assumption of Paulter (Poulter, 1969) that the Humboldt penguin uses echolocation when catching fish, perceiving sounds arising from cavitation processes in water jets flowing around the bird's body.

Many migratory birds migrate at night, and it is possible that at least some of them acoustically estimate their flight altitude or find high obstacles in the way. This seems all the more likely that night migrants constantly emit various sound signals when flying in conditions of limited visibility (Thorpe, 1961, Schwartzkopff, 1962). In particular, there are reports that curlews using echolocation can navigate in fog, using special signals for this purpose (Freeman, 1950).

E.SH. AIRAPETYANTS A.I. KONSTANTINOV. ECHOLOCATION IN NATURE. Publishing house "SCIENCE", LENINGRAD, 1974

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