86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



quently. They joined and followed individuals of other species rela- 

 tively more frequently than did the sooty-capped bush-tanagers in the 

 same area (in the same patches of shrubbery), and were joined and 

 followed by individuals of other species relatively less frequently 

 than were the sooty-capped bush-tanagers. The role of the black- 

 cheeked warblers in the mixed flocks of this area appeared to be only 

 slightly nuclear, and almost exactly intermediate between passive and 

 active. 



In another area, slightly above 7,000 feet, sooty-capped bush-tana- 

 gers were comparatively rare, and black-cheeked warblers appeared to 

 be joined and followed by individuals of other species relatively more 

 frequently than in the area below 7,000 feet. All the species which 

 joined and/or followed the black-cheeked warblers above 7,000 feet 

 also occurred in the same area as the black-cheeked warblers below 

 7,000 feet. This would suggest that black-cheeked warblers may take 

 the place of sooty-capped bush-tanagers in mixed flocks in areas 

 where the latter are rare or absent (and brown-capped bush-tanagers 

 are absent). 



There was no evidence that black-cheeked warblers have a special 

 interspecific preference for sooty-capped bush-tanagers, or that the 

 latter have a special interspecific preference for the former in either 

 of the areas where the two species were seen together. It is con- 

 ceivable, therefore, that one of these species has become more like 

 the other, or that they are convergent, in voice and appearance, in 

 order to facilitate associations with other species. It may be advan- 

 tageous for the black-cheeked warbler and/or the sooty-capped bush- 

 tanager to be joined and/or followed by individuals of other species. 

 If associations with other species are advantageous to the black- 

 cheeked warbler, then it may be advantageous for it to resemble the 

 sooty-capped bush-tanager because individuals of other species tend 

 to join and follow sooty-capped bush-tanagers. If associations with 

 other species are advantageous to the sooty-capped bush-tanager, it 

 may be advantageous for it to resemble the black-cheeked warbler be- 

 cause individuals of other species tend to join and follow black- 

 cheeked warblers. If associations with other species are advantageous 

 to both the black-cheeked warbler and the sooty-capped bush-tanager, 

 the similarities between them may tend to increase the frequency 

 and/or speed with which individuals of other species join and follow 

 both of them, as individuals of other species may become conditioned 

 more rapidly and easily to the essentially single set of stimuli actually 

 presented by both the black-cheeked warbler and the sooty-capped 



