NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN I3I 



show a high degree of intraspecific gregariousness can probably cross 

 such wide gaps and occupy new areas successfully more frequently 

 than individuals of montane species that do not show a high degree of 

 intraspecific gregariousness because they will often arrive in a new 

 area on the other side of a wide gap in a group, from which mates can 

 be obtained, while individuals of other species will usually arrive singly 

 and be unable to obtain mates, as they will seldom or never be able 

 to attract other individuals of their own species from adjacent areas, 

 and other individuals of their own species will not be likely to stray 

 into the new area of their own accord. 23 



The migrant species, which occur in mixed flocks in the Tropics 

 only part of the year, have not been mentioned in the preceding dis- 

 cussion because they are a very special case. It might be noted, how- 

 ever, that most of them are quite widely distributed when in the 

 Tropics, and tend to occur in a relatively wide variety of habitats. 

 They all seem to be primarily joiners and/or followers when they 

 associate with mixed flocks in the Tropics, either attendant species or 

 not very important active nuclear species. They also tend to choose 

 a relatively wide variety of companions of other species. 



Published accounts of mixed flocks of tanagers, honeycreepers, 

 and/or finches (with or without warblers) would suggest that such 

 flocks are not evenly distributed throughout tropical America. More 

 different types of mixed flocks of these birds have been noted in the 

 lowlands of Central America and southern Brazil than in the low- 

 lands of the Amazon basin, in spite of the fact that there are prob- 

 ably more genera and species of lowland tanagers and honeycreepers 

 in the Amazon basin than anywhere else in the American Tropics. 

 Similarly, more different types of mixed flocks of these birds have 

 been noted in the mountains of Central America than in the central 

 Andean region, in spite of the fact that there are more genera and 

 species of montane tanagers, honeycreepers, and finches in the cen- 

 tral Andes than anywhere else in the mountains of the American 

 Tropics. 



The greater number of records of mixed flocks in Central America 

 and southern Brazil may be partly an artifact, as there have been 

 more observers interested in avian behavior and ecology in these re- 



23 The distribution of montane species in much of tropical America is essen- 

 tially insular. It has often been remarked that species of land birds that show 

 a high degree of intraspecific gregariousness are more likely to cross water gaps 

 and establish themselves successfully on islands than are species of otherwise 

 similar birds that do not show a high degree of intraspecific gregariousness (see, 

 for instance, Mayr, 1931, and Amadon, 1950). 



