NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN II7 



It may be significant, in this connection, that black-throated tanagers 

 were relatively rare in the area near Iquitos (the birds that associated 

 with the silver-billed tanagers were the only black-throated tanagers I 

 saw in this area), while the silver-billed tanagers were comparatively 

 common. It is possible that this area is not a very favorable environ- 

 ment for black-throated tanagers, or that the ones I saw had just 

 moved into the area and may have been partly dependent upon the 

 silver-billed tanagers' ability to discover food sources (see below). 



DISCUSSION 



THE ADVANTAGES OBTAINED BY MEMBERSHIP IN MIXED 



FLOCKS 



All or almost all the authors who have discussed the functions of 

 mixed flocks have suggested that birds become associated in such flocks 

 in order to get food, to get protection from enemies, and/or to satisfy 

 some gregarious motivation or instinct (see the summary in Rand, 

 1954). All three suggestions may be correct. It should be noted, how- 

 ever, that the three functions are not strictly commensurable. 



The habit of associating in mixed flocks may be an adaptation to 

 get food and/or protection, but it can hardly be described as an adapta- 

 tion, in the same sense of the term, to satisfy a gregarious instinct. 

 From an evolutionary point of view the development of a gregarious 

 instinct that can be satisfied by association in mixed flocks is probably 

 a means to an end, not an end in itself. 16 A bird may join individuals 

 of other species, or allow itself to be joined by individuals of other 

 species, because such associations satisfy its gregarious instincts ; but 

 such instincts probably have been evolved, in all or most cases, be- 

 cause interspecific gregariousness provides certain concrete advan- 

 tages. Gregariousness seems to be a type or method of adaptation, 



10 In ethological terms, association with individuals of other species may be 

 considered a "consummatory situation" for any bird that derives a definite 

 satisfaction from such an association. Searching for individuals of other species 

 to become associated with may be considered a form of "appetitive behavior." 

 (See discussions of these terms in Tinbergen, 195 1; Hinde, 1953; and Bastock, 

 Morris, and Moynihan, 1953.) Unfortunately, there have been almost no ana- 

 lytical studies of the motivations impelling an individual of one species to 

 become associated with individuals of other species. It is possible that individ- 

 uals of some species have some sort of interspecific gregarious drive, which can 

 be satisfied only by associations with individuals of other species, quite apart 

 from any intraspecific gregarious drive or gregarious aspects of other instincts; 

 but this has certainly not yet been proved to exist. 



