NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 91 



or family group comes very close to another, as happens very fre- 

 quently, one (or more) of the birds usually utters a moderately loud, 

 twittering, greeting call. During obviously hostile territorial disputes 

 between neighboring yellow-thighed finches, the disputing birds usu- 

 ally utter harsher, louder versions of the same twittering call. These 

 hostile calls sometimes appear to be attractive to individuals of several 

 other species, in addition to the bush-tanagers cited above. 



None of the other members of the montane bush alliances is as gen- 

 erally and consistently conspicuous as yellow-thighed finches. The 

 conspicuous characters of yellow-thighed finches contribute greatly to 

 the general conspicuousness of the mixed flocks with which they are 

 associated, and sometimes seem to attract individuals of other species 

 to themselves. It is possible that the production of either one or both 

 of these effects is the primary function of these characters, i.e., the 

 principal adaptive advantage they were originally evolved to secure. 

 If so, this would be a rather unusual specialization. None of the 

 other Panamanian finches, tanagers, or honeycreepers seems to have 

 developed as extreme a group of conspicuous characters as an adapta- 

 tion to stimulate the formation and maintain the cohesion of mixed 

 flocks but not unmixed flocks. 



It should be noted that the geographical distribution of yellow- 

 thighed finches is not very wide (see below). It seems unlikely that 

 they are more gregarious among themselves in other areas than they 

 are on the Volcan de Chiriqui, where they are very abundant. 



Yellow-thighed finiches are sometimes joined and/or followed by 

 individuals of other species with some appreciable frequency; but 

 it is probably only in certain flocks composed of yellow-thighed 

 finches and yellow-throated bush-finches alone that they are ever con- 

 sistently joined and followed by individuals of another species more 

 frequently than they join and follow individuals of other species. 



THE EFFECTS OF THE TERRITORIAL BEHAVIOR OF CERTAIN SPECIES 



UPON CERTAIN MIXED FLOCKS 



It has already been mentioned several times that individuals of sev- 

 eral different species are attracted by the hostile calls of yellow-thighed 

 finches and/or sooty-capped bush-tanagers, and that mated pairs and 

 family groups of many territorial species associate with mixed flocks. 

 These factors interact in a rather interesting way in mixed montane 

 bush flocks just below 7,000 feet on the western slopes of the Volcan 

 de Chiriqui, where both sooty-capped bush-tanagers and yellow- 

 thighed finches are common. 



