NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN IO7 



creeper alliance, in favor of retaining the habit of performing sup- 

 planting attacks. The frequent performance of both interspecific and 

 intraspecific supplanting attacks is probably a primitive character, as 

 such attacks are often performed by individuals of many species that 

 are not very gregarious. 



Andrewartha and Birch (1954) have suggested that competition 

 between animals of different species is seldom very significant. Their 

 discussion of this subject is not, perhaps, very convincing (see Lack, 

 1954, and comments by Brown and Wilson, 1956, and Hutchinson, 

 1959). In any case, it should be emphasized that the relations between 

 many members of mixed blue and green tanager and honey creeper 

 flocks are particularly clear examples of competition. The competition 

 between such birds is often as overt as possible. 



Members of higher-altitude mixed montane bush flocks compete di- 

 rectly with one another less frequently than do members of mixed blue 

 and green tanager and honeycreeper flocks. Individuals of different 

 species in higher-altitude mixed montane bush flocks try to feed on 

 the same foods at the same times less frequently than do individuals 

 of different species in mixed blue and green tanager and honeycreeper 

 flocks. Individuals of the higher-altitude montane bush alliances 

 would probably obtain extra food less frequently by the performance 

 of supplanting attacks than do individuals of the blue and green tana- 

 ger and honeycreeper alliance. This may be the reason why the advan- 

 tages of performing many interspecific supplanting attacks seem to 

 be outweighed by the disadvantages of such behavior in higher-alti- 

 tude mixed montane bush flocks (the disadvantages presumably being 

 the same as in mixed blue and green tanager and honeycreeper flocks). 



(It is possible that the total amount of competition, both direct and 

 indirect, between species of the higher-altitude montane bush alliances 

 is less than the total amount of competition between species of the 

 blue and green tanager and honeycreeper alliance ; but this would also 

 be very difficult, if not impossible, to prove.) 



The differences between the interspecific hostile reactions of the 

 species of the higher-altitude montane bush alliances and those of the 

 species of the blue and green tanager and honeycreeper alliance seem 

 to be intrinsic at the present time. They seem to be due to internal 

 differences between the birds themselves, not their different environ- 

 ments. Birds of the higher-altitude montane bush alliances may oc- 

 cur in social situations that seem to be essentially similar to those in 

 which birds of the blue and green tanager and honeycreeper alliance 

 perform many interspecific supplanting attacks. They may even be- 



