Io8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



come involved in such situations rather frequently (but less frequently, 

 on the average, than birds of the blue and green tanager and honey- 

 creeper alliance). But they still do not usually perform interspecific 

 supplanting attacks in such situations. This would indicate that the 

 reduced frequency of interspecific supplanting attacks by members 

 of the higher-altitude montane bush alliances is a result of basic 

 changes in their internal hostile drives (and/or their "innate" respon- 

 siveness to hostile stimuli) in the course of evolution. Interestingly 

 enough, the intraspecific hostile reactions of all or most species of 

 the higher-altitude montane bush alliances do not seem to have be- 

 come reduced to the same extent in the course of evolution. Indi- 

 viduals of all or most of these species become engaged in disputes with 

 other birds of the same species approximately as frequently as do 

 members of the blue and green tanager and honeycreeper alliance 

 which show a comparable degree of intraspecific gregariousness. 



The frequency of interspecific supplanting attacks in mixed blue 

 and green tanager and honeycreeper flocks may help to explain why 

 the social roles of most of the nuclear species of these flocks are so 

 clear cut and one sided, and why some species have developed un- 

 usually dull plumage. Because interspecific supplanting attacks tend 

 to reduce the cohesion of mixed flocks, the species of the blue and 

 green tanager and honeycreeper alliance have probably been subjected 

 to particularly strong selection pressures in favor of developing 

 mechanisms to minimize some of the effects of interspecific supplant- 

 ing attacks and/or restore the cohesion of flocks as rapidly as possi- 

 ble after interspecific supplanting attacks. Both the dull plumage 

 and/or the very one-sided nature of the social roles of some species 

 may help to accomplish these objectives. They may both permit or 

 facilitate particularly rapid "friendly" reactions within the mixed blue 

 and green tanager and honeycreeper flocks. 



The species of the higher-altitude mixed montane bush flocks may 

 not have been subjected to strong selection pressures in favor of the 

 development of similar characters simply because the cohesion of 

 their flocks is seldom subjected to the sudden shocks of interspecific 

 supplanting attacks. 



SIMPLER MIXED FLOCKS 



Three other types of mixed flocks that have been studied much 

 less intensively than blue and green tanager and honeycreeper flocks 

 or the mixed montane bush flocks may be described very briefly, as 



