NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN J$ 



Volcan de Chiriqui. Above approximately 6,800 feet the brown- 

 capped bush-tanager is more or less abruptly replaced by the sooty- 

 capped bush-tanager, and the collared redstart becomes more common 

 than the slate-throated redstart. In spite of their different composi- 

 tions, most of the higher-altitude flocks, both above and below 6,8oo 

 feet, are essentially very similar in structure. 



They are usually comparatively stable, more so than most mixed 

 blue and green tanager and honeycreeper flocks. Individuals of all the 

 species occurring in these montane bush flocks do tend to join and 

 leave one another frequently, or are joined and left by one another 

 frequently ; but there are indications that individuals of some of these 

 species tend to remain together with individuals of other species con- 

 tinuously for longer periods of time, on the average, than do individ- 

 uals of any of the species of the blue and green tanager and honey- 

 creeper alliance. Some of the higher-altitude mixed montane bush 

 flocks are also long sustained as groups in spite of the fact that the 

 individual birds in such flocks may change from time to time. This 

 may be clarified by an example. A pair of birds of a strongly terri- 

 torial species may remain associated with a mixed montane bush flock 

 as long as the flock remains within the pair's territory and then drop 

 out of the flock when the flock moves on ; but this pair's place in the 

 flock may be taken, immediately, by another pair of the same species, 

 so that the specific composition of the flock is not changed by the 

 change of individual birds. 



The comparative stability of the higher-altitude mixed montane 

 bush flocks seems to be correlated with certain other distinctive fea- 

 tures of social behavior within such flocks. Interspecific joining and 

 following reactions are comparatively common, more so than in all 

 or most mixed blue and green tanager and honeycreeper flocks, while 

 interspecific supplanting attacks are comparatively rare, much less 

 common than in all or most mixed blue and green tanager and honey- 

 creeper flocks. 



The frequencies of some interspecific reactions in some higher-alti- 

 tude montane bush flocks may be illustrated by the figures in table 45. 

 These figures are counts of the number of times individuals of certain 

 species were seen to be involved in interspecific joining, following, and 

 supplanting reactions in higher-altitude montane bush flocks during 

 approximately 25 hours of intermittent observations between Octo- 

 ber 4 and October 10, i960, on the Volcan de Chiriqui. The method of 

 observation used was the same as in the counts of reactions in mixed 

 blue and green tanager and honeycreeper flocks, except that each in- 



