NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 83 



hostile calls in response to sooty-capped bush-tanagers. They react to 

 sooty-capped bush-tanagers much less frequently and less strongly 

 than might be expected in view of the fact that the two species may 

 occupy the same habitats, are rather similar in physical appearance, 

 and have very similar repertories of calls, display movements, and pos- 

 tures. The few definite social reactions of brown-capped bush- 

 tanagers to sooty-capped bush-tanagers probably occur when the 

 former mistake the latter for suboptimal members of their own 

 species. 



Brown-capped bush-tanagers are more responsive to yellow-thighed 

 finches, which are very different from brown-capped bush-tanagers in 

 appearance and have some rather different habits, but are common 

 throughout the range of brown-capped bush-tanagers on the western 

 slopes of the Volcan de Chiriqui. 



Brown-capped bush-tanagers react to the hostile calls of yellow- 

 thighed finches more frequently and more vigorously than they do 

 to those of sooty-capped bush-tanagers. I have seen single brown- 

 capped bush-tanagers fly considerable distances to join groups of 

 yellow-thighed finches that were disputing among themselves and 

 uttering many hostile calls. These brown-capped bush-tanagers usually 

 uttered many hostile calls of their own after joining the yellow-thighed 

 finches, sometimes continuing to utter these calls as long as the yellow- 

 thighed finches continued disputing among themselves. These inci- 

 dents occurred near the upper border of the range of brown-capped 

 bush-tanagers on the western slopes of the Volcan de Chiriqui, in 

 which area these birds are relatively rare. It is possible, therefore, 

 that the single brown-capped bush-tanagers that reacted so vigorously 

 to the hostile calls of yellow-thighed finches did so because they were 

 unable to associate with and perform their usual hostile behavior with 

 other individuals of their own species. Their hostile motivation may 

 have "accumulated" because it could not be vented in its usual way, 

 and then "overflowed" upon suboptimal objects that would not have 

 released hostility in other circumstances. The reactions of these single 

 brown-capped bush-tanagers were particularly interesting, neverthe- 

 less, because the hostile calls of yellow-thighed finches are very dif- 

 ferent from any calls of brown-capped bush-tanagers and because the 

 same brown-capped bush-tanagers that reacted to the hostile calls of 

 yellow-thighed finches usually ignored all the calls of sooty-capped 

 bush-tanagers in the same neighborhood. 



I once heard a single territorial brown-capped bush-tanager near 

 the upper border of the range of the species utter many hostile calls 



