I36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



extra food and protection from predators, by associating in mixed 

 flocks. The additional protection from predators may be the most 

 important advantage obtained by most of these species when they as- 

 sociate in mixed flocks in most circumstances. 



The initial step in the evolution of most highly organized mixed 

 flocks of passerine birds was probably the formation of special social 

 bonds between a species that showed a high degree of intraspecific 

 gregariousness and one or more species that did not show a high 

 degree of intraspecific gregariousness. The former probably became 

 a passive nuclear species in most cases. The subsequent history of 

 such flocks was probably largely dependent upon the appearance and 

 behavior of the other species that became attached to the passive 

 nuclear species. Some of the characters of some species that seem 

 to be adaptations to promote the formation and/or maintain the 

 cohesion of a particular type of mixed flock may facilitate the in- 

 corporation of additional species as new regular members in mixed 

 flocks of that particular type; but other characters that seem to be 

 adaptations to subserve the same functions may tend to prevent the 

 incorporation of additional species. 



The finches and tanagers that are known to play passive nuclear 

 roles in mixed flocks in the lowlands of central Panama or other 

 lowland areas of the American Tropics are less widely distributed 

 than species that are known to play active nuclear roles in the same 

 mixed flocks (and some closely related species that do not play any 

 sort of nuclear role in mixed flocks). Active nuclear species of finches 

 and tanagers that are adapted to the lowland Tropics may be able to 

 invade new areas within the American lowland Tropics more success- 

 fully, in most cases, than otherwise similar but passive nuclear species. 



Two largely passive nuclear species of the mixed montane bush 

 flocks on the Volcan de Chiriqui are much more widely distributed 

 than most of the other species commonly occurring in such flocks. 

 The comparatively wide distribution of these passive nuclear species 

 may be due to the fact that they both show a high degree of intra- 

 specific gregariousness. On logical grounds, it might be expected 

 that montane species that show a high degree of intraspecific gregari- 

 ousness would be able to invade new montane areas in the American 

 Tropics more successfully, on the average, than otherwise similar 

 species that do not show a high degree of intraspecific gregariousness. 



Various observations during this study, and published records, 

 would suggest that mixed flocks of tanagers, honeycreepers, and/or 

 finches are not evenly distributed throughout tropical America. It is 



