NO. 5 THE YELLOW-RUMPED TANAGER — MOYNIHAN 27 



The differences in numbers of displays between yellow-rumped 

 tanagers and other highly gregarious tanagers may be correlated with, 

 i.e. causally related to, differences in breeding behavior. All the 

 other highly gregarious tanagers whose behavior has been studied 

 in detail tend to feed and move about in groups, but they do not 

 show the tendency to breed close together which is so conspicuous in 

 yellow-rumped tanagers. Behavioral interactions between birds 

 breeding in colonies or semi-colonial groups may be particularly 

 complex (simply because the constant presence of other individuals 

 will stimulate many incompatible tendencies, such as sex and hostility, 

 that are stronger on the average during the breeding season than at 

 other times of the year). A great variety of signal patterns may be 

 necessary in order to regulate or control such complex interactions 

 with maximum efficiency. (Another factor of slightly different order 

 may be involved. Colonial or semi-colonial breeders such as yellow- 

 rumped tanagers may have developed, or retained, comparatively 

 strong aggressive tendencies in order to prevent their strong gregari- 

 ous tendencies from leading to overcrowding or general promiscuity. 

 There is evidence that a high degree of aggresiveness in itself favors 

 the development or retention of a great variety of displays, irrespective 

 of the social structure in which the aggressiveness is expressed. 

 See Moynihan, 1963.) 



3. The intergrading of the vocal patterns. 



The major vocal patterns of yellow-rumped tanagers intergrade 

 with one another more frequently and more conspicuously than the 

 corresponding patterns of any other American "nine-primaried" song- 

 bird whose behavior has been studied. This seems to be a highly 

 specialized character and must be adaptive. The only other groups 

 of animals in which a similar contrast between species with intergrad- 

 ing vocal patterns and species with distinct, nonintergrading patterns 

 has been recognized is the mammalian order of primates. Among 

 primates there is evidence to suggest that the species with intergrading 

 vocal patterns are those which are least dependent upon vocal signals 

 alone for the regulation of their social behavior. Intermediate (and 

 ambivalent) notes, produced by intergrading, have certain advantages 

 as signals. They may convey complex messages (i.e. the precise 

 combination of tendencies in the animal producing the notes) in very 



surprisingly large number of different ways, e.g. the combinations of one 

 vocalization with several different types of feather-raising and of one type of 

 feather-raising with several different types of vocalization. Different combina- 

 tions of display components seem to be more common in the repertory of 

 yellow-rumped tanagers than in those of many or most related species. 



