NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 115 



ance in certain special circumstances. The same is true of dusky-faced 

 tanagers. When such encounters occur, most of the members of the 

 blue and green tanager and honeycreeper alliance, except crimson- 

 backed tanagers (see below), tend to ignore the dusky-tailed ant- 

 tanagers and dusky- faced tanagers in much the same way that they do 

 yellow-rumped tanagers. (Slud, i960, has already noted that dusky- 

 faced tanagers are usually ignored by birds of other species in Costa 

 Rica.) It seems likely that many or most members of the blue and 

 green tanager and honeycreeper alliance have developed special aver- 

 sions to dusky-tailed ant-tanagers and dusky-faced tanagers as well 

 as yellow-rumped tanagers. 



Such aversions are quite different from hostile interspecific prefer- 

 ences, such as those of green honeycreepers and shining honeycreepers 

 for red-legged blue honeycreepers. The aversions are revealed by a 

 conspicuous lack of overt responses, while the hostile interspecific 

 preferences are expressed by supplanting attacks and/or other overt 

 aggressive acts. 



The habitat preference of yellow-rumped tanagers may also impede 

 associations with other species in another way. Most members of the 

 blue and green tanager and honeycreeper alliance (again with the ex- 

 ception of crimson-backed tanagers) are reluctant to remain in the 

 scrub along the edges of rivers for any considerable length of time. 

 They seldom remain in this environment for more than a few seconds 

 or minutes before flying to trees or other scrub. This behavior effec- 

 tively neutralizes the joining and following tendencies of yellow- 

 rumped tanagers, simply because the latter are usually very reluctant 

 to leave the river-bank scrub. 



Although yellow-rumped tanagers do not play a significant social 

 role in any highly integrated or very complex mixed flocks in central 

 and eastern Panama at the present time, they may be partly or com- 

 pletely preadapted to do so. It is easy to imagine how a relatively 

 slight change in the habitat preference of yellow-rumped tanagers, or 

 the appearance in the region of a new species that did not have a 

 special aversion to them, or the loss of the special aversion by one of 

 the species that already occurs in the region, would allow the yellow- 

 rumped tanagers to play a more important nuclear role in mixed flocks 

 (which might appear to be highly integrated from the very beginning) . 



(Crimson-backed tanagers occasionally associate quite closely with 

 dusky-tailed ant-tanagers in certain scrub areas. Young crimson- 

 backed tanagers just out of the nest seem to be more likely to form 

 such associations than older birds. Such young birds may well be de- 



