NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 1 35 



curring in mixed blue and green tanager and honeycreeper flocks have 

 also developed other special characters, such as dull "neutral" plumage, 

 to facilitate the performance of their characteristic social roles within 

 mixed flocks. 



The species that occur in mixed blue and green tanager and honey- 

 creeper flocks can be divided into "regular" and "occasional" members 

 of the flocks. They can also be divided into "nuclear" and "attendant" 

 species. A species may be considered nuclear in any given type of 

 mixed flock if its behavior contributes appreciably to stimulate the 

 formation and/or maintain the cohesion of that particular type of 

 mixed flock. Some species are nuclear in one type of mixed flock but 

 not in others. The nuclear species of mixed blue and green tanager 

 and honeycreeper flocks can be divided into "passive" nuclear species 

 and "active" nuclear species. Passive nuclear species are usually 

 joined and followed by other species much more frequently than they 

 join and follow other species. Active nuclear species usually join and 

 follow other species much more frequently than they are joined and 

 followed by other species. 



The mixed montane bush flocks that occur at higher altitudes on 

 the Volcan de Chiriqui seem to be approximately as complex and as 

 highly organized as mixed blue and green tanager and honeycreeper 

 flocks; but their social structure is not identical with that of the 

 latter in all details. Interspecific supplanting attacks are much less 

 common in the higher-altitude mixed montane bush flocks than in 

 mixed blue and green tanager and honeycreeper flocks. Some of the 

 nuclear species of higher-altitude mixed montane bush flocks are not 

 consistently passive or consistently active. Most of the differences be- 

 tween the social organization of the higher-altitude mixed montane 

 bush flocks and the social organization of mixed blue and green 

 tanager and honeycreeper flocks may be due to the fact that the mem- 

 bers of higher-altitude mixed montane bush flocks compete directly 

 with one another for food less frequently than do members of mixed 

 blue and green tanager and honeycreeper flocks. 



Several other types of mixed flocks, including flocks of small 

 finches, forest birds on an isolated mountain in central Panama, and 

 species of the genus Ramphocelus were studied in less detail. These 

 flocks appeared to be simpler and/or less highly organized than mixed 

 blue and green tanager and honeycreeper flocks or the higher-altitude 

 mixed montane bush flocks on the Volcan de Chiriqui. 



It seems likely that most species of neotropical finches, tanagers, 

 honeycreepers, and/or warblers obtain several advantages, including 



