NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 105 



that these blue tanagers really do have the same gregarious tendencies 

 as lowland blue tanagers in central Panama.) 



Similar factors may help to explain the social isolation of two resi- 

 dent species of the genus Piranga on the Volcan de Chiriqui, the 

 white-winged tanager (P. leucoptera) and the flame-colored tanager 

 (P. bidentata). Individuals of both species occur in some of the 

 same areas as many regular members of the mixed montane bush 

 flocks ; but they seem to be associated with such flocks even less f re- 

 quently than are blue tanagers. They obviously do not have the 

 special joining reactions of summer tanagers. They also seem to be 

 definitely unattractive (at least) to all or most of the species com- 

 monly occurring in mixed montane bush flocks. This may be partly 

 due to the fact that adult male white-winged tanagers and flame- 

 colored tanagers are largely bright red or orange (and immature males 

 of both species frequently show traces of the same colors). 



SIGNIFICANCE OF SOME OF THE PRINCIPAL DIFFERENCES 

 BETWEEN MIXED BLUE AND GREEN TANAGER AND HONEY- 

 CREEPER FLOCKS AND THE HIGHER-ALTITUDE MIXED 

 MONTANE BUSH FLOCKS 



Among the more obvious differences between the higher-altitude 

 mixed montane bush flocks (which seem to be the most highly, or 

 most thoroughly, organized type of mixed montane bush flock) and 

 the mixed blue and green tanager and honeycreeper flocks are the 

 following : 



1. All or most of the members of a mixed blue and green tanager 

 and honeycreeper flock are usually confined to a rather narrow level 

 of vegetation (usually the treetops) at any given time, while the mem- 

 bers of higher-altitude mixed montane bush flocks are frequently 

 scattered among many different levels of vegetation at the same time. 



2. Supplanting attacks by individuals of one species upon indi- 

 viduals of other species are relatively common in mixed blue and green 

 tanager and honeycreeper flocks, but relatively very rare in higher- 

 altitude mixed montane bush flocks. 



3. The nuclear members of mixed blue and green tanager and 

 honeycreeper flocks are usually completely passive or completely ac- 

 tive, while most of the nuclear species of higher-altitude mixed mon- 

 tane bush flocks can be either passive or active or may play an inter- 

 mediate role. 



4. Mixed blue and green tanager and honeycreeper flocks usually 

 include species that have developed an apparently highly specialized 



