NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 85 



at a time. (This is presumably correlated with the lesser degree of 

 intraspecific gregariousness shown by sooty-capped bush-tanagers.) 

 It is very obvious that they are joined and followed by individuals 

 of other species much less frequently when they are silent, even when 

 they are very active, than when they are vocal, even when they are 

 not very active. The hostile calls of sooty-capped bush-tanagers, a 

 sound very much like the corresponding calls of brown-capped bush- 

 tanagers, also seem to be considerably more attractive to individuals of 

 other species than many or all of their nonhostile calls (see below). 



Sooty-capped bush-tanagers are sometimes attracted by the hostile 

 calls of yellow-thighed finches in very much the same way as are 

 brown-capped bush-tanagers. (I have not seen sooty-capped bush- 

 tanagers react to the hostile calls of brown-capped bush-tanagers, per- 

 haps because brown-capped bush-tanagers were relatively rare in the 

 areas where I saw the two species together.) 



The sooty-capped bush-tanager may thus be considered a passive 

 nuclear species ; but it is less effective as such than the brown-capped 

 bush-tanager. 



THE BASILEUTERUS WARBLERS 



Black-cheeked warblers were not as common as bush-tanagers in 

 the areas where mixed montane bush flocks were studied most inten- 

 sively; but they were observed frequently enough to discover their 

 usual social role in at least some types of mixed flocks. 



Black-cheeked warblers seem to be approximately as gregarious 

 among themselves as are sooty-capped bush-tanagers, which they 

 resemble in several ways, but are more strictly confined to moderately 

 low shrubbery. They are very much like sooty-capped bush-tanagers 

 in physical appearance. The two species are similar in shape, and 

 their plumage looks very much the same at a distance in the field. Both 

 species appear to be big headed and plump bodied, generally olive in 

 color, with blackish heads marked by light chins and whitish lines 

 above and behind the eyes. Black-cheeked warblers are less frequently 

 vocal than sooty-capped bush-tanagers ; but their most common vocal 

 patterns, including sharp call notes and rattles, are reminiscent of 

 both the bush-tanagers. These similarities are not likely to be coin- 

 cidental. They may be some form of mimicry. 



In one area on the Volcan de Chiriqui, just below 7,000 feet, where 

 sooty-capped bush-tanagers were quite common, black-cheeked war- 

 blers joined and followed individuals of other species, and were also 

 joined and followed by individuals of other species moderately fre- 



