78 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



reminiscent of the calls of some thicket-inhabiting finches (but not 

 the finches usually occurring in mixed montane bush flocks on the 

 Volcan de Chiriqui). The squirrels may tend to enchance the con- 

 spicuousness and the attractiveness of the mixed bird flocks with 

 which they are associated. ) 



Brief observations of the lower-altitude mixed montane bush flocks 

 on the Volcan de Chiriqui would suggest that they are usually more 

 loosely organized than the higher-altitude mixed montane bush flocks. 

 It has already been mentioned that the lower-altitude mixed montane 

 bush flocks frequently include more species than the higher-altitude 

 flocks ; but they usually include relatively few individuals of the spe- 

 cies that play the most important nuclear roles in the higher-altitude 

 flocks. The structure of the lower-altitude mixed flocks is similar to 

 that of the higher-altitude mixed flocks insofar as the members of the 

 lower-altitude flocks may also occur at many different levels of vegeta- 

 tion; but the associations between most of the common species of the 

 lower-altitude flocks seem to be briefer, on the average, than the asso- 

 ciations between most of the common species of the higher-altitude 

 flocks. Interspecific joining and following reactions seem to be com- 

 paratively rare in the lower-altitude mixed montane bush flocks ; but 

 interspecific supplanting attacks are comparatively common, much 

 more so than in the higher-altitude mixed montane bush flocks, and 

 perhaps as common as in mixed blue and green tanager and honey- 

 creeper flocks. 



THE ROLES OF DIFFERENT SPECIES WITHIN MIXED FLOCKS 



BROWN-CAPPED BUSH-TANAGER 



The usual social role of brown-capped bush-tanagers in mixed spe- 

 cies flocks, especially at moderately high altitudes, is very similar to 

 that of plain-colored tanagers in blue and green tanager and honey- 

 creeper flocks. 



Brown-capped bush-tanagers are frequently joined and followed by 

 individuals of many other species, much more frequently than they 

 themselves join and follow individuals of other species. 



As in the case of the plain-colored tanagers, the usual social role 

 of brown-capped bush-tanagers in mixed flocks seems to be largely 

 or completely a consequence of their social behavior apart from mixed 

 flocks. Some of the intraspecific social behavior patterns of the two 



