134 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



tent, where they provide competition for birds of South American ori- 

 gin. The forest and scrub areas of lowland Central America are partly 

 separated from the forests of the Amazonian region at the present time 

 by the Andes of northwestern South America and some other physio- 

 graphic barriers, while the montane forest and scrub of Central 

 America are separated from the Andes by intervening lowland areas 

 such as the Atrato River valley and central Panama. Some or all of 

 the barriers partly separating Central American faunas from South 

 American faunas may have been less important, or may not have 

 existed at all, during some of the climatic fluctuations of the Pleisto- 

 cene ; but they have certainly been intermittently effective, at least 

 slowing the spread of many species of birds, for a considerable length 

 of time (see Chapman, 191 7 and 1926). 



It is possible, therefore, that species of tanagers, honeycreepers, 

 and neotropical finches that have developed the habit of associating 

 in mixed flocks in any capacity or social role may tend to survive 

 better in relatively unfavorable or partly isolated habitats and/or 

 invade relatively unfavorable or partly isolated habitats more suc- 

 cessfully than related and otherwise similar species that have not 

 developed such habits. 



SUMMARY 



Several different types of mixed species flocks, composed of finches, 

 tanagers, honeycreepers, and/or warblers (and sometimes species of 

 other groups as well) are common in Panama. Two types of these 

 flocks were studied in some detail : mixed blue and green tanager and 

 honeycreeper flocks in the lowlands of central Panama, and mixed 

 montane bush flocks on the Volcan de Chiriqui in western Panama. 



Mixed blue and green tanager and honeycreeper flocks are com- 

 plex societies. Each of the species commonly occurring in such 

 flocks tends to play a characteristic and rather consistent social role, 

 which is more or less distinctly different from that of every other 

 species commonly occurring in the flocks. Such roles are the results 

 of complex interactions between each species and at least one (usually 

 several) other species. Several of the species tend to react differ- 

 ently to each of several other species. Some species seem to have 

 developed "special interspecific preferences" for certain other species. 

 Special interspecific preferences may be expressed by apparently 

 "friendly" joining and following reactions and/or by largely or com- 

 pletely hostile supplanting attacks. Some of the species commonly oc- 



