included in these phrases. On the other hand the screams of the young 

 birds (nestlings ), -which indicate only one thing - hunger - are narrowly 

 specialized, stable, and do not carry any additional connotations, such 

 as geographical information. Besides, there is no biological justifica- 

 tion for the latter. Thus, the coincidence of geographical (popula- 

 tional) variability with certain elements in the voice of birds has an 

 adaptive character and is the result of an important evolutionary ac- 

 quisition - the narrow specialization of separate elements. 



Until recently the problems of the acoustics of associations did 

 not arouse any particular interest. In our opinion they are of con- 

 siderable interest, since each of the mechanisms of vocal communication 

 is formed not only under the influence of one's own species and physical 

 environmental conditions but is also influenced by the general biogenous 

 sound background, which is characteristic of the habitat in which the 

 species evolves. 



The physical foundations of the acoustics of natural zones and land- 

 scapes have also been insufficiently investigated, and our information 

 is scant. The greatest differences are noticeable in the case of 

 "closed" and "open" habitats. Thus, for instance, in the tundra the 

 distance a voice carries is limited by air humidity, but the weak ab- 

 sorption of sound by the extremely humid soil is a compensatory factor. 

 In the forest the distance a voice carries is greatly reduced owing to 

 the strong absorption and multiple reverberation of sound by tree trunks, 

 branches and leaves. In steppes the development of the grass cover 

 creates the conditions of a "closed" landscape for many animals. In the 

 desert, the sound -absorbing properties of sandy soils are unfavorable to 

 the propagation of sound. Another phenomenon with a similar effect is 

 the particularly active air movement produced by constant winds, con- 

 vection of heated layers, etc. Similar conditions influencing the 

 propagation of sound also exist in the various biotopes. 



It is therefore natural that such marked differences in conditions 

 exert a considerable influence on the mechanisms of vocal communication. 

 Species which have become adapted to life in open spaces tend toward a 

 generally limited reliance on sound signals, and utilize visual cues to 

 a greater extent. This can be easily observed in Ratitae, Otididae, 

 Cicoriformes, sandpipers, cranes, etc. 



Observations have shown that the nature of vocal communication may 

 serve as an indication of the place of origin, development and further 

 expansion of a group. It can also indicate the antiquity of its' link with 

 a particular landscape and type of association. 



The influence of the biogenous background on sound signalization is 

 also subject to the laws of geographical variation. It is closely re- 

 lated to the structure and stratigraphy of associations. The following 

 changes in the role of the biogenous background in the formation of sound 

 signalization can be observed from north to south, from the tundras 

 towards zones of optimal life conditions. 



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