In the creation of optimal territorial relations within a popu- 

 lation, the most important role falls to the song, which is the most 

 adaptable, changeable, and individually variable aspect of the voice 

 (Mal'chevskii, 1958; 1959; Kistyakovskii, 1958; Smogorzhevskii, 1950). 

 These characteristics of the song, together with its identif icational 

 functions, are at the same time an essential specific adaptation that 

 indicates, on one hand, the delimitation of the nest territory, and, on 

 the other, serves to attract the female and thus assumes a role in 

 sexual selection. 



The delineation of territory is of the greatest importance in 

 groups with pronounced nesting conservatism. In this case the need 

 arises for a permanent "fixation" [the staking of a claim], which sig- 

 nifies the occupation of the territory by the group. This is achieved 

 by certain modes of behavior, and in the event of extremely well- 

 developed hearing and voice mechanisms, sound becomes the most important 

 means for supporting and maintaining the territorial structure of a 

 species. Birds, which are not provided with a we 11- developed sense of 

 smell, nor with glands that excrete odiferous substances, as in the 

 case of insects and mammals, have vocal-communication systems developed 

 to absolute perfection. The importance of sounds serving as signals 

 increases with the distribution of the species in "closed," easily 

 overlooked biotopes, where sound becomes practically the only certain 

 means of communication and maintenance of contact. Sound orientation 

 acquires an even greater importance under conditions of high population 

 density; or, conversely, when the population is dispersed. 



The role of the voice as a means of identification is particularly 

 important in the case of birds with antiphonal song (Cisticola, 

 Trachyphonus , Laniarius ) that live in dense thickets, where the role of 

 visual signals is reduced to a minimum. All the principal behavior 

 patterns (recognition of the mate, nuptial display, patrol of the ter- 

 ritory, etc.) are achieved here almost exclusively by means of the voice. 

 Each pair of birds living under the above conditions has its own song 

 pattern (Thorpe, 1963). 



Similar conditions bring about the intensif ication^of vocal "commu- 

 nication in both sexes in other orders. For instance, in owls the nec- 

 essity for vocal communication is apparently caused not only by the 

 limited visibility under conditions of "closed biotopes" but is also 

 related to their nocturnal way of life (Kistyakovskii, 1958). In the 

 cuckoo the same phenomenon is due to its polygamous way of life. Some- 

 times it is not the male but the female who clearly establishes her 

 individuality by making a loud call, which causes her to stand out 

 against the background of surrounding sounds . The male then seeks her 

 out by her voice (as described for painted snipes, but ton- quails, phal- 

 aropes, and some species of Anatidae, by Kistyakovskii, 1958). 



By mastering and using the signals of other species, some of them 

 even belonging to different classes, species which specialize in the 

 imitation of the elements of their sound environment elaborate the 



