124 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



serine birds may be as follows. The first stage is probably usually 

 the formation of specialized social bonds between a species that shows 

 a high degree of intraspecific gregariousness and one or a few other 

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

 The former species is almost certain to become a passive nuclear 

 species in the evolving flock. The subsequent history of the flock is 

 probably largely dependent upon the nature of the other species that 

 first become attached to the passive nuclear species. If these species 

 become attendant species, the flock may not develop much further. 

 If one or more of them should become an active nuclear species, 

 however, this may provide a strong impetus to further elaboration 

 of the flock. Active nuclear species will greatly increase the con- 

 spicuousness of the flock and contribute a variety of new stimuli, new 

 sounds, colors, visual patterns, and/or movements, to broaden and 

 strengthen the general effect which the flock produced upon other 

 species in its vicinity. These additions may increase the attractive- 

 ness of the mixed flock to other species and induce the other species 

 to become regular members of the flock in turn. If one or more of 

 these new regular members should also become active nuclear species, 

 this may provide another strong impetus to further elaboration of 

 the flock. Thus, the flock may grow by a snow-balling process until 

 all the suitable species in the area and environment have become in- 

 corporated in it in one way or another. 



One of the later stages in the development of many large and com- 

 plex types of mixed flocks may be the fusion of smaller types of mixed 

 flocks, each of which grew up around a different passive nuclear spe- 

 cies. It is possible, for instance, that the typical large blue and green 

 tanager and honeycreeper flocks in Panama were developed by an in- 

 complete fusion of predominantly tanager flocks and predominantly 

 honeycreeper flocks. 



The evolution of very specialized morphological and behavioral 

 characters, such as neutral coloration, social mimicry, and the reduc- 

 tion or loss of interspecific aggressiveness, by some or all of the regu- 

 lar members of the flock, may also be typical of the later stages in 

 the development of many mixed flocks. Neutral coloration and the 

 reduction of interspecific aggressiveness probably facilitate the incor- 

 poration of more new members in the flock ; but social mimicry may 

 have just the opposite effect. As social mimicry will tend to make the 

 members of a flock more and more homogeneous in appearance 

 and/or voice, it will probably make it more and more difficult for other 

 species of dissimilar appearance or voice to join the flock. Social 



