Il8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



while food and protection are advantages that may be obtained by the 

 adaptation. 



It seems likely that many or most members of many or most mixed 

 flocks obtain both extra food and added protection, more or less simul- 

 taneously, by their associations with one another ; but the relative im- 

 portance of the two advantages is probably very different for different 

 species and for individuals of the same species at different times. This 

 certainly seems to be true in the case of most members of the mixed 

 flocks studied in Panama. 



Some observers of tropical mixed flocks (e.g., Chapin, 1932; Rand, 

 1954; Slud, i960; and Swynnerton, 1915) have suggested that they 

 are primarily feeding associations, that all or most birds become as- 

 sociated with such flocks primarily or exclusively because they tend to 

 get more food when they are in mixed flocks than when they are not. 

 This generalization seems to have been derived from observation of 

 mixed flocks of birds that are primarily or exclusively insectivorous 

 and live inside tropical forests (these were the first tropical mixed 

 flocks to be studied). 



Some of the primarily or exclusively insectivorous birds in mixed 

 blue and green tanager and honeycreeper flocks and mixed montane 

 bush flocks, e.g., summer tanagers, Wilson's warblers, red-faced spine- 

 tails, and the redstarts, probably obtain feeding advantages by asso- 

 ciating with mixed flocks, and maintain such advantages nearly con- 

 tinuously as long as they remain within the flocks, in much the same 

 way as do insectivorous birds inside tropical forests. The other mem- 

 bers of mixed flocks must at least frequently serve as beaters for 

 some of the insectivorous birds. 



There are many indications, however, that the food factor is less 

 important in the case of many other members of many mixed blue 

 and green tanager and honeycreeper flocks and mixed montane bush 

 flocks. The primarily frugivorous and/or nectarivorous birds prob- 

 ably do not obtain feeding advantages from their associations with 

 mixed flocks as frequently or as consistently as do the insectivorous 

 birds. It is often obvious that they are not helping one another to get 

 food. It is difficult, in fact, to imagine how frugivorous and/or 

 nectarivorous birds that are territorial or confined to definite home 

 ranges could derive any considerable feeding advantages by associat- 

 ing with mixed flocks as long as they remain in or near their usual 

 territories or ranges. Such birds are usually thoroughly familiar with 

 the actual and potential sources of fruit and nectar in and near their 



