NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 8l 



tanagers involved. As soon as this outburst occurred, the brown- 

 capped bush-tanagers were usually joined by individuals of other spe- 

 cies, such as yellow-thighed finches or red-faced spinetails, who had 

 been ignoring them until they became very noisy. The disputes 

 between the brown-capped bush-tanagers usually subsided after a few 

 minutes. The disputing birds either separated or joined up with one 

 another to form a more or less amicable flock. In either case, some 

 or all of the brown-capped bush-tanagers were usually followed for 

 long periods of time after the end of the dispute by some or all of the 

 individuals of other species that had been attracted by the sound of the 

 dispute. 



(Occasionally, but relatively very rarely, a brown-capped bush- 

 tanager is also attracted by the sound of other brown-capped bush- 

 tanagers uttering threat calls during a dispute. It may then fly to 

 join the disputing birds and utter threat calls itself and/or actually 

 begin to fight with the other birds. This would suggest that the sound 

 of threat calls by one brown-capped bush-tanager usually or always 

 stimulates both the attack and escape drives of other brown-capped 

 bush-tanagers that hear the calls. The escape drives of the birds that 

 hear such calls are probably usually or always stimulated more 

 strongly than their attack drives ; but their attack drives are apparently 

 stimulated strongly enough to be expressed by overt activity in some 

 cases. 



There are indications that the reactions of individuals of other 

 species that are attracted by the sounds of the threat calls of brown- 

 capped bush-tanagers are seldom or never produced by the same moti- 

 vation as the similar reactions of other brown-capped bush-tanagers. 

 Individuals of other species that are attracted by the sounds of the 

 threat calls of brown-capped bush-tanagers seldom or never perform 

 hostile patterns themselves after joining the brown-capped bush- 

 tanagers. Their reactions seem to be usually or always essentially 

 "friendly." They apparently recognize the threat calls of brown- 

 capped bush-tanagers as calls of a species with which they are accus- 

 tomed to associate ; but they seldom or never react as if they under- 

 stood the hostile significance of such calls.) 



The reactions of individuals of other species to the threat calls of 

 brown-capped bush-tanagers might suggest that the usual social role 

 of the latter in mixed flocks is largely determined by their vocal pat- 

 terns rather than their movements or their physical appearance; but 

 the effect of their vocal patterns is probably reinforced by some or all 

 of their other characteristics. It is possible, for instance, that the 



