NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 120, 



than are individuals of passive nuclear species. Individuals of active 

 nuclear species moving into a new area probably tend to join and fol- 

 low individuals of many already-established species, including passive 

 nuclear species, other active nuclear species, and less specialized 

 species. Individuals of passive nuclear species moving into a new area 

 are probably not joined and followed by individuals of as many al- 

 ready established species. They may attract active nuclear species and 

 less specialized species, but probably not other passive nuclear species. 

 Individuals of active nuclear species may also tend to react to indi- 

 viduals of already-established species more frequently and more rap- 

 idly than individuals of already-established species react to individuals 

 of passive nuclear species. By associating more closely with the 

 already-established species, individuals of active nuclear species prob- 

 ably discover the food sources and/or potential danger spots of an 

 area with which they are unfamiliar more rapidly than do individuals 

 of passive nuclear species in similar circumstances. 



There may also be a general, but indirect and much less consistent, 

 correlation between the ranges and social roles of the species of the 

 montane bush alliances of Panama. Interestingly enough, this cor- 

 relation seems to be almost the reverse of the correlation noted in the 

 case of the tanagers and finches of the lowland flocks. 



The brown-capped bush-tanager, the most important passive nuclear 

 species of the higher-altitude montane bush flocks, and a species that 

 shows a high degree of intraspecific gregariousness, is very widely 

 distributed in montane and hill regions from southern Mexico to 

 northern Argentina. The golden-crowned warbler, which also shows 

 a high degree of intraspecific gregariousness and may play an equally 

 important passive nuclear role in the lower-altitude montane bush 

 flocks, is also very widely distributed from northeastern Mexico to 

 eastern and southern Brazil. 



None of the species of the montane bush alliances that usually or 

 always tend to join and/or follow other species very frequently are 

 as widely distributed as the brown-capped bush-tanager or the golden- 

 crowned warbler. Some of them, including species that are usually 

 active nuclear species and those that are usually attendants in mixed 

 montane bush flocks, have comparatively very restricted ranges. The 

 yellow-thighed finch, the ruddy tree-runner, the black-cheeked war- 

 bler, and the collared redstart, are restricted to the mountains of 

 Panama and Costa Rica ; and several others do not extend very much 

 farther into the mountains of northern Central America and/or north- 



