NO. 7 FLOCKS OF NEOTROPICAL BIRDS MOYNIHAN II 



relationships are widespread among most of the species of the blue and 

 green tanager and honeycreeper alliance. 



It is difficult to believe, in any case, that birds of different species 

 could occur together as frequently as many members of the blue and 

 green tanager and honeycreeper alliance without forming social bonds 

 among themselves. 



Many or most of the birds in mixed blue and green tanager and 

 honeycreeper flocks may be attracted to one another as a result of some 

 conditioning process, having "learned" that the presence of other spe- 

 cies is usually an indication of food in the neighborhood. Birds of the 

 blue and green tanager and honeycreeper alliance may also tend to se- 

 cure more food more easily, in the long run (although not always), 

 when in mixed flocks than when not in mixed flocks. It is even possible 

 that facilitation of feeding is the principal adaptive advantage of the 

 habit of forming mixed flocks, i.e., the principal reason why such 

 habits were selected during the evolution of many species of the blue 

 and green tanager and honeycreeper alliance (but see comments be- 

 low). Even if all these possibilities should be established as proven 

 facts, however, they would not be evidence that the attraction of food 

 is the sole cause of the formation and maintenance of many or most of 

 the associations between individuals of different species in most of 

 the mixed blue and green tanager and honeycreeper flocks observed 

 at the present time. 



There are even some indications that some slight social attraction 

 between individuals of different species is very widespread among 

 passerine birds in general, even species that are seldom or never found 

 in mixed flocks. Observation of many different species of passerine 

 birds in Panama would suggest that an individual of almost any 

 species may approach, or at least perform intention movements of 

 approaching, individuals of almost any other species, especially upon 

 first hearing or seeing the other individuals. This may be nothing 

 more than "curiosity" ; but it does tend to bring birds together, and it 

 may be one of the sources from which more strongly gregarious 

 tendencies have been developed in the course of evolution (see below). 



All other factors being equal, the chances of one bird's approaching 

 another bird of another species seem to be dependent upon the degree 

 of resemblance between the two birds. Birds of different species are 

 more likely to approach one another if they resemble one another in 

 certain conspicuous features, such as color or voice, than if they are 

 very different from one another in all features. 



Although food may not be the only, or the principal, attraction 



