88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



Some other species of the montane bush alliances tend to perform 

 many more interspecific joining reactions than interspecific following 

 reactions (see below). The marked contrast between certain species 

 that are primarily followers and others that are primarily joiners is 

 one of the most characteristic features of mixed montane bush flocks, 

 which are quite different from mixed blue and green tanager and 

 honeycreeper flocks in this respect. All or most of the active nuclear 

 and attendant species in mixed blue and green tanager and honey- 

 creeper flocks cannot be classified as either followers or joiners in the 

 same way as some species of the mixed montane bush flocks. 



All other factors being equal, individuals of all or most species 

 would probably usually prefer to follow rather than join individuals 

 of other species. By following, a bird can associate with another bird 

 without coming too close, which might provoke a dispute. In some 

 circumstances, however, following tends to be more difficult than 

 joining. The ease or difficulty of following or joining is often largely 

 dependent upon the nature of the vegetation in which such reactions 

 occur. 



Yellow-thighed finches may be able to follow so much more fre- 

 quently than to join because they are essentially birds of thickets and 

 low shrubbery. They sometimes go fairly high into trees, 20 or 30 

 feet above the ground, but only in certain exceptional situations. They 

 seldom or never do so except when there is a thick curtain of shrub- 

 berylike vegetation extending continuously or nearly continuously 

 from the ground up to a higher level, e.g., when there are thick tangles 

 of vines extending from just above the ground to the higher branches 

 of trees. 



Individuals of other species that also occur in thickets and low 

 shrubbery on the Volcan de Chiriqui also tend to perform more inter- 

 specific following reactions than interspecific joining reactions as long 

 as they remain in thickets and low shrubbery. This is true of the bush- 

 tanagers and black-cheeked warblers, as well as individuals of several 

 less passive species (see below). On the other hand, individuals of 

 at least some active species of the montane bush alliances tend to per- 

 form more interspecific joining reactions than interspecific following 

 reactions when they are in high shrubbery and trees (see below). 

 They may also perform relatively more interspecific joining reactions 

 when they are in high shrubbery and trees than when they are in low 

 shrubbery and thickets. 



The explanation of all or most of these differences is fairly obvious. 

 Low shrubbery and thickets tend to be denser than high shrubbery and 



