106 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



type of dull plumage to facilitate interspecific gregariousness ; but 

 none of the species of the higher-altitude mixed montane bush alli- 

 ances seems to have evolved a specialized plumage to subserve similar 

 functions. 



These differences between the two types of flocks are presumably 

 adaptive. They may also be causally related to one another. 



It has already been mentioned that mixed blue and green tanager 

 and honeycreeper flocks are usually restricted to one rather narrow 

 level of vegetation because most of the species occurring in such 

 flocks are usually restricted to one and the same rather narrow level 

 of vegetation. Higher-altitude mixed montane bush flocks often 

 extend through several different levels of vegetation because they 

 frequently include species that prefer different levels of vegetation 

 and/or frequently move back and forth between several different 

 levels. (The usual restriction of most of the species of the blue and 

 green tanager and honeycreeper alliance to one rather narrow level 

 of vegetation may be an indication that most of these species are more 

 narrowly specialized, restricted to narrower ecological niches, than 

 many or most of the species of the higher-altitude montane bush alli- 

 ances ; but this would be extremely difficult, if not impossible, to 

 prove.) 



The members of mixed blue and green tanager and honeycreeper 

 flocks are not only usually restricted to one rather narrow level of 

 vegetation, but also frequently feed on the same foods. Thus they 

 tend to compete with one another in a very direct way. This may 

 help to explain why interspecific supplanting attacks are so relatively 

 frequent in mixed blue and green tanager and honeycreeper flocks. 



The performance of supplanting attacks within a flock is probably 

 often disadvantageous because such attacks tend to reduce the cohe- 

 sion of the flocks in which they occur ; but this disadvantage must be 

 outweighed, in the case of many or all of the members of the blue 

 and green tanager and honeycreeper alliances by compensatory ad- 

 vantages. One of these is probably increased success in competition 

 for food. Individuals of many species of the blue and green tanager 

 and honeycreeper alliance probably obtain more food by performing 

 supplanting attacks than they would if they did not do so. A bird that 

 performs a supplanting attack upon another bird of the same or a dif- 

 ferent species, when the other bird is approaching food or actually eat- 

 ing, will usually force the other bird to retreat, at least temporarily, 

 and may then be able to appropriate the food itself. It seems likely, 

 therefore, that there have been strong selection pressures, during the 

 evolution of many species of the blue and green tanager and honey- 



