Under tundra conditions the degree of sound saturation of the en- 

 vironment is rather limited, and its formative influence therefore insig- 

 nificant. The ability to see great distances in the surroundings, to- 

 gether with a weakly developed sound background, results in a simple song 

 pattern in autochthonous tundra birds. With other morphological indices 

 as well, differentiation between single individuals and pairs depends on 

 the distance at which communication can be continuously maintained by 

 vocal contact. 



Proceeding southward one finds that, together with the increasing 

 complexity of the environment, the structure and stratigraphy of asso- 

 ciations also become more complicated. Interspecific relations acquire 

 greater importance, as does, in particular, the natural sound background, 

 which plays an increased role in the formation of mechanisms of vocal 

 contact. The biogenous sound background reaches its highest degree of 

 saturation in the so-called "bird markets," i.e., in mixed forests con- 

 sisting of coniferous and broadleaved trees, and in broadleaved and 

 tropical forests. When speaking of associations inhabiting these types 

 of forests, it is possible to speak of the objective existence of an 

 involved acoustic structure. There arises here a definite need for the 

 strictest form of animal adaptation, since sound signals have to be 

 easily recognized and differentiated by their frequency, intensity, 

 pitch, modulation and general pattern, over and above the general choral 

 background. Should this fail to be achieved, the birds are in danger 

 of having their means of orientation and communication greatly reduced. 

 The problem is to find a "free area" in the acoustic field, that is, 

 one not filled with signals of the same parameters. In addition, the 

 importance of other means of orientation in the forest, such as visual 

 perception, becomes greatly reduced. With a weakly developed sense of 

 smell, sound communication becomes the only really certain means for 

 maintenance of contacts. All this makes it difficult to find such a 

 "free area," which we suggest calling an "acoustic niche." 



Data obtained from recording and analyzing voices already indicate 

 the existence of such niches. The peculiarities of sound signalization 

 in the Turdidae family are sufficient to give an idea of how originally 

 and rationally vocal communications are organized among these birds. 



In our opinion, comparisons of the biogenous sound backgrounds of 

 various faunistic complexes of high tasonomic rank are of special in- 

 terest. They will undoubtedly show correlations and parallelisms of 

 which vie as yet have only the vaguest idea. Investigations in that 

 direction will be of great help not only for discovering the laws of 

 formation and evolution of the mechanisms of vocal communications, but 

 also will perhaps help in defining certain aspects of the general char- 

 acter of evolution with greater precision. 



However, data on the bioacoustics of associations have not only 

 an exclusively scientific interest. During recent years their great 

 practical importance has been demonstrated. This is clearly illustrated 

 by the practical application of such knowledge to associations in oceans 



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