NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 121 



tion with individuals of other species in mixed flocks may provide 

 most of the advantages that could be obtained by association with 

 other individuals of the same species in an unmixed flock, without the 

 disadvantages of membership in an unmixed flock. 



THE ORIGIN OF HIGHLY INTEGRATED MIXED FLOCKS 



On logical grounds, one might expect that highly integrated mixed 

 flocks could develop by extension and "regularizing" of occasional as- 

 sociations between species that are closely related to one another phylo- 

 genetically, 18 such as the crimson-backed and yellow-rumped tanagers, 

 and/or by strengthening and specialization of the social bonds between 

 less closely related species that may occur together in larger but essen- 

 tially casual aggregations, such as the montane forest groups on Cerro 

 Campana. There is some evidence that the latter process may have 

 been more important than the former in the evolution of most highly 

 integrated mixed flocks of passerine birds. 



Occasional and loose associations between species that are closely 

 related to one another phylogenetically are common in many areas. 

 The reasons for this are obvious. Closely related species often have 

 similar habits and often occur in the same general habitats when they 

 occur in the same areas. Closely related species are also often similar 

 in appearance, at least in shape or some details of their plumage pat- 

 terns. Unless inhibited or counteracted by other factors, such simi- 

 larities will inevitably tend to promote or facilitate associations be- 

 tween closely related species whenever they come into social contact 

 with one another. Regular and close associations between closely re- 

 lated species seem to be much less common among passerine birds. 

 Closely related (i.e., congeneric) species of passerine birds seem to be 

 linked by specialized social bonds less frequently than are less closely 

 related species. This may also be a result of competition. 



As a general rule, different species of birds that occupy the same 

 areas and habitats can only compete with one another to a certain 

 limited extent if they are to continue to live side by side. One species 

 will usually or always replace another (or others) in any given area 

 and habitat unless there is some sort of (at least partial) ecological 

 isolation between the species. The principle of "competitive exclu- 

 sion," i.e., the principle that "complete competitors cannot coexist" 



18 It will be necessary in some parts of the following discussion to use the 

 terms "related" and "relationships" in two different senses, to refer to both 

 phylogenetic and social relationships. The type of relationship discussed in any 

 given passage will be specified, or should be clear from the context. 



