128 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



of their most important associates, as Slud does not mention which 

 species played active nuclear roles in the flocks he observed. He does, 

 however, state that the green honeycreeper, which is an active nuclear 

 species in central Panama, also occurs in mixed flocks in Costa Rica. 

 Three of the four passive nuclear species cited by Slud are the only 

 species of their genera in Costa Rica (and most of Central America) ; 

 but there are two other species of Tachyphonus in Costa Rica, the 

 white-shouldered tanager and the white-lined tanager, that do not 

 seem to play nuclear roles in the Costa Rican mixed flocks, any more 

 than they do in mixed blue and green tanager and honeycreeper flocks 

 in central Panama (see above). It may be significant, therefore, that 

 both the white-shouldered tanager and the whilte-lined tanager are 

 very widely distributed, much more so than the closely related but 

 passive nuclear tawny-crested tanager. The white-shouldered tanager 

 and the white-lined tanager extend over all or most of Amazonian 

 South America. 



(Both sexes or the adult males of the four passive nuclear species 

 cited by Slud are largely black and/or yellow or yellow-olive. Sev- 

 eral other tanagers and some migrant warblers that Slud observed 

 associating with mixed flocks are also prominently marked with black 

 and/or yellow. Such similarities are presumably not coincidental and 

 may facilitate the formation of mixed flocks in much the same way 

 as the similar colors of many members of the montane bush alliances.) 



Finally, variable seedeaters, which are very gregarious among them- 

 selves, are much less widely distributed than blue-black grassquits, 

 which are much less gregarious among themselves but tend to join 

 and follow variable seedeaters in mixed flocks of small finches. 



These facts would suggest that there may be a very general rule, 

 among many different types of lowland tanagers and finches in tropi- 

 cal America, that species that play passive nuclear roles in mixed 

 flocks have comparatively narrow ranges, while species that play ac- 

 tive nuclear roles have very broad ranges. 



Within the lowlands of the American Tropics, the occupation of 

 new areas may be easier for individuals of active nuclear species 

 adapted to lowland tropical habitats than for individuals of passive 

 nuclear species that are otherwise equally well adapted to such habitats. 

 Individuals of the two types of species may reach new areas equally 

 frequently, on the average, and may be almost equally likely to secure 

 mates in new areas (see below) ; but individuals of active nuclear spe- 

 cies moving into a new area are probably much more likely to form 

 strong social bonds with other species already established in the area 



