114 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 143 



more striking example of the same phenomenon or, perhaps more 

 probably, an example of definite interspecific "aversion." 



The characters of yellow-rumped tanagers that seem to be adapted 

 to promote intraspecific gregariousness are somewhat more extreme 

 or exaggerated than the corresponding characters of crimson-backed 

 tanagers (the "flash" patterns of yellow-rumped tanagers are even 

 more conspicuous than those of crimson-backed tanagers, and yellow- 

 rumped tanagers are probably even noisier, on the average, than 

 crimson-backed tanagers) ; but yellow-rumped tanagers seem to at- 

 tract individuals of most species of the blue and green tanager and 

 honeycreeper alliance much less frequently than do crimson-backed 

 tanagers, even in the most favorable circumstances. Individuals of 

 several species that are regular members of the blue and green tana- 

 ger and honeycreeper flocks (including plain-colored tanagers, palm 

 tanagers, blue tanagers, and golden-masked tanagers) sometimes come 

 down to the shrubbery along the edges of rivers, if there are no trees 

 nearby. At such times they usually ignore the yellow-rumped tana- 

 gers almost completely. Even palm tanagers seem to join and follow 

 yellow-rumped tanagers relatively very rarely. 



It is conceivable that most members of the blue and green tanager 

 and honeycreeper alliance tend to ignore yellow-rumped tanagers 

 simply because the latter are so distinctive in voice and appearance. 

 To the human eye and ear, however, yellow-rumped tanagers are not 

 more different from most of the members of the blue and green tana- 

 ger and honeycreeper alliance than are crimson-backed tanagers. It is 

 perhaps more likely, therefore, that most of the members of the blue 

 and green tanager and honeycreeper alliance have developed a special- 

 ized aversion to yellow-rumped tanagers, a special power of resistance 

 to the attraction of their restlessness and conspicuousness. This aver- 

 sion may have been developed in order to avoid being sucked down 

 into low shrubbery too frequently. There are indications that associa- 

 tions with any species that is largely confined to low scrub and thickets 

 may be disadvantageous for most members of the blue and green 

 tanager and honeycreeper alliance. 



Two other scrub-inhabiting species, the dusky-tailed ant-tanager 

 and the dusky-faced tanager (Mitrospingus cassinii), are common in 

 parts of central and eastern Panama. Both are noisy, restless, rather 

 gregarious among themselves, and less distinctively colored than either 

 yellow-rumped tanagers or crimson-backed tanagers. It has already 

 been mentioned that dusky-tailed ant-tanagers sometimes encounter 

 many members of the blue and green tanager and honeycreeper alii- 



