NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 89 



trees, and twigs and branches that birds can use as perches tend to be 

 much closer together in thickets and low shrubbery than in high shrub- 

 bery and trees. Birds moving through thickets and low shrubbery can 

 usually regulate their distances from one another more or less at will, 

 because they can move by very short stages, hopping or flying between 

 perches that are only a few inches apart. Such birds can usually follow 

 one another without joining whenever they want to. Birds moving- 

 through trees do not always have the same freedom of choice. They 

 may have to move from tree to tree by comparatively long flights 

 because the trees are not very close together, and may not be able to 

 regulate their distances from one another as precisely as can birds in 

 thickets and low shrubbery. They may have to bunch up in certain 

 particular trees, i.e., perform joining reactions, if they are to keep in 

 touch with one another at all. 15 



In the case of yellow-thighed finches there is also another factor 

 involved. These birds in thickets and low shrubbery are often at- 

 tracted to individuals of other species that are moving through higher 

 vegetation more or less distinctly separated by a layer of open space 

 from the underlying low vegetation. In such circumstances the yellow- 

 thighed finches are usually prevented from trying to join the individ- 

 uals of other species by their reluctance to move through the open 

 space. 



Although yellow-thighed finches will follow and join individuals of 

 almost any other species, they seem to prefer to follow brown-capped 

 bush-tanagers when the latter are common, sooty-capped bush-tana- 

 gers when brown-capped bush-tanagers are rare or absent, and black- 

 cheeked warblers at high altitudes where brown-capped bush-tanagers 

 are absent and sooty-capped bush-tanagers are rare or absent. 



The marked special interspecific preference of yellow-thighed 

 finches for brown-capped bush-tanagers is also shown in several other 

 ways. Yellow-thighed finches are often attracted by the hostile as well 

 as the nonhostile vocal patterns of brown-capped bush-tanagers. They 

 do not seem to react as positively to the hostile calls of any other 

 species as they sometimes do to those of brown-capped bush-tanagers. 



Yellow-thighed finches also tend to follow brown-capped bush- 



15 The frequencies of interspecific reactions by members of the blue and green 

 tanager and honeycreeper alliance are also affected by density of vegetation. 

 Individuals of most species of this alliance tend to perform relatively more 

 interspecific following reactions in environments such as the clearing on Barro 

 Colorado Island, where the treetops form a continuous band of vegetation 

 around the edge of the clearing, than in environments such as the areas near 

 Gamboa and Frijoles, where trees are more scattered. 



